Full text of "The age of the earth"

See other formats

HARPER'S LIBRARY of LIVING THOUGHT

THE AGE

THE EARTH

ARTHUR HOLMES
B.Sc., A.R.C.S.

HARPER

BROTHERS

LONDONXNEWYOEK

FIG. i.— Haloes in cleavage flake of Biotite. X 70.

FIG. 2. — Radium Halo (lower part of field) and Thorium Halo (upper

part of field) in Biotite cut across cleavage. X 100.
FIG. 3. — Haloes in cleavage flake of Biotite. X 85.
FIG. 4. — Haloes due to Ra. emanation (inner disc), Ra. A and Ra. B. X45O.

PLEOCHROIC HALOES

THE AGE OF THE
EARTH

ARTHUR HOLMES

B.Sc., A.R.C.S.

MEMBER OF THE IMPERIAL COLLEGE

FELLOW OF THE GEOLOGICAL SOCIETY OF LONDON
FELLOW OF THE ROYAL GEOGRAPHICAL SOCIETY

ILLUSTRATED WITH TWENTY FIGURES AND DIAGRAMS

LONDON AND NEW YORK
HARPER fcT BROTHERS

45 ALBEMARLE STREET, W.
1913

Published March, 1913

PROFESSOR THE HON. R. J. STRUTT

AND

PROFESSOR W. VV. WATTS

IN GRATITUDE

FOR EARLY TRAINING

AND ADVICE

PREFACE

IT is perhaps a little indelicate to ask of our
Mother Earth her age, but Science acknow-
ledges no shame and from time to time has boldly
attempted to wrest from her a secret which is
proverbially well guarded. On January 30th, 1911,
I placed before the Natural History Society of the
Royal College of Science a brief account of some
of these attempts, and out of that paper this little
book has gradually developed. In the present
survey of the subject — the measurement of geo-
logical time — I have endeavoured to give as full
and complete an account of the chief methods of
attack as space would allow. My particular object
has been to draw attention to their respective
limitations and to test the validity of the various
assumptions on which they are based. So far,
the most remarkable feature of the problem lies
in the extraordinary discrepancy between the
conclusions drawn from the two most prominent
methods of dealing with it. I have sought to
mitigate the severity of this disagreement by dis-
cussing the possibility and consequences of recon-
ciliation and by suggesting a path along which
reconciliation may be found. If my treatment
should stimulate a greater interest in the time-
problem, or provide material for further discussion,
it cannot fail to bring nearer the ultimate solution

PREFACE

of a question which for more than a century has
had an unrivalled notoriety for provoking con-
troversy.

I desire to acknowledge my gratitude to Prof.
J. Joly and the Council of the Royal Dublin
Society for permission to use the admirable series
of micro-photographs which constitute the Frontis-
piece ; to Prof. P. Lowell for his photographs of
the Polar Caps of Mars ; and to Sir Wm. Crookes
and the Council of the Royal Society for the
photograph and radiograph of a piece of pitch-
blende.

To a number of friends who have helped me in
revising the proof-sheets, and to others who have
turned my rough sketches into finished diagrams,
I wish to tender my thanks.

Finally, I have a special pleasure in expressing
my gratitude for the valuable assistance which has
been afforded me by my friend Mr. R. W. Law-
son, of Armstrong College, Newcastle-upon-Tyne.
While the manuscript was in preparation I had
the benefit of his suggestions and friendly criticism,
and at a later stage he kindly verified many of my
calculations. I am indebted to him in particular
for the mathematical treatment of the final section
of Appendix A (dealing with the uranium time-
average) and for the two diagrams with which it
is illustrated.

ARTHUR HOLMES.

GEOLOGICAL DEPARTMENT,

IMPERIAL COLLEGE, S.W.

December ist, 1912.

CONTENTS

I'AGE

PREFACE ........ ix

LIST OF ILLUSTRATIONS . . . . . xii

CHAP.

I. THE TIME PROBLEM AND ITS HISTORY i
II. TIME AND TIDE IN THE SOLAR SYSTEM 22

III. ASTRONOMICAL CONSIDERATIONS . -32

IV. THE WORK OF DENUDATION . . 48
V. THE SALINITY AND AGE OF THE OCEANS 61

.VI. SEDIMENTATION AND GEOLOGICAL TIME. 76

VII. RADIOACTIVITY 91

VIII. THE THERMAL ENERGY OF THE SUN . no

IX. THE THERMAL ENERGY OF THE EARTH 122

X. RADIOACTIVE MINERALS AND THEIR AGES 137

XI. REVIEW OF THE EVIDENCE . . .166

APPENDIX A -177

APPENDIX B 184

INDEX 191

LIST OF ILLUSTRATIONS

Figures printed as plates
FIG. i \

FIG* 2 | pleochroic Haloes .... Frontispiece
FIG! 4 J

FACING PAGE

FIG* 6 } Spiral Nebulae 28

FIG* 8 / Polar Caps °f Mars 39

FIG' n I Phot°graPhs of Pitchblende . . .92

Figures printed in text

PAGE

FIG. 9. Glacial Movement in S. Scandinavia . . .40

FIG. 10. Upper Cretaceous Strata of Colorado . . • 45
FIG. II. Section illustrating the Formation of Sediments

on Continental Shelf 83

FIG. 14. Apparatus for Estimating Radium . . . 106

FIG. 15. Pleochroic Haloes of the Uranium Family . 108

FIG. 16. Temperature Gradients 127

FIG. 17. Geological Time Scales 172

APPENDIX A—

Graph I 180

Graph II 181

Table of Radioactive Elements 190

xii

THE AGE OF THE EARTH

CHAPTER I
THE TIME PROBLEM AND ITS HISTORY

Primitive races and their creation myths — The chrono-
logies of Eastern sages — The demands of geology opposed
by theological prejudice — Bishop Ussher's date of the
creation of the world — The Deluge and the Doctrine
of Catastrophism — Ludicrous conceptions of fossils —
The rise and eventual success of Uniformitarianism —
Leonardo da Vinci — Steno — Generelli — Desmarest —
Hutton — Scrope — Lyell — Ussher's chronology rejected —
— Geological time no longer limited — The hour-glass of
denudation and deposition — Darwin's excessive drafts on
time — The conservation of energy — Geology limited once
again — Thomson on the age of the earth and sun, 1862 —
The great controversy — Huxley, 1869 — Perry and the
correspondence in Nature, 1895 — Sir G. Darwin and the
birth of the moon — Clarence King's estimate of the age
of the earth — Time implied by the stratified rocks — Joly
on the salinity and age of the oceans — The significance
of radioactivity — Emission of energy by radium and its
widespread distribution — Radioactivity provides a new
measure of time — Controversy again — Methods of dealing
with the problem — The inadequate testimony of biology.

most primitive races of mankind, strenu-
JL ously engaged in the daily struggle for
existence, appear to have given little thought to
the antiquity of the world on which they lived.
Even at the present day there exist barbaric tribes
to whom it has never occurred that the earth may
have had a beginning. The conception of creation,

THE AGE OF THE EARTH

of the production of order from chaos, indicates
a marked intellectual advance, but into the myths
and legends of which this idea was so often the
inspiration, the element of time did not usually
enter.

The East African natives have traditions of the
upheaval of mountains and of the freeing of the
earth from bondage, legends which may have had
a foundation in the phenomena exhibited by
volcanoes and glaciers. If the question " When ? "
is asked, the natural reply is merely a look of
astonishment, and persistent enquiry elicits nothing
more definite than a vague ' ' Long ago . ' ' Anything
else, of course, could not be expected from a people
whose conceptions of the development of the uni-
verse are limited to a recognition of the alternation
of seasons and the insistent nrysteries of life and
death.

The priests and philosophers who flourished
during the ancient civilizations of the East,
speculated in greater detail and devoted some
attention to the elaboration of .a chronology of
earth-history. The Chaldeans had a well-deserved
reputation for astronomy, and their known
observations go back for more than 6000 years.
Cicero relates that their venerable priesthood had
records of stellar observations stretching back
for 470,000 years ; a fanciful period which tallies
with the date assigned by the Chaldeans to the
origin of Man. According to the same remarkable
system, the earth had already existed for 215
myriads * of years. The Persian sages, led by

* A myriad = 10, ooo.

TIME PROBLEM AND ITS HISTORY

Zoroaster believed that the total duration of the
world's existence was limited to 12,000 years.
The Hebrew chronological tables are familiar to
everyone, and the restricted interpretation which
was placed upon them during the Middle Ages,
when they affected European beliefs so powerfully,
will be presently referred to. Opposed to these
ideas of a definite beginning were the more abstract
conceptions of Indian philosophers, who regarded
Time and the Earth as eternal.

To determine the age of the earth, or to express
the actual problem with more accuracy, to measure
the duration of geological time, became a definite
scientific aspiration only during the last century.
The ultimate purpose of Geology is essentially to
elucidate the history of the earth, a record of
which is imperfectly written in the stratified and
igneous rocks to which access is possible. As the
characters and sequence of the various formations
gradually became better understood, it was found
that the story they disclosed was one of successive
changes of life and scene of the most impressive
kind. The immensity of time which seemed to
be indicated was at first a fruitful source of
confusion and prejudice, for it brought geology
into disrepute at an early period, owing to the
widely prevalent idea that the writings of Moses
fixed the antiquity of the universe beyond dispute.
It is indicated by a marginal reference in most
English Bibles that the creation of the world took
place in the year 4004 B.C. This famous estimate,
which probably represents the most limited period
ever assigned to the past duration of our planet,
was put forward in 1650 by Bishop Ussher. Some

THE AGE OF THE EARTH

such date as this had been generally believed in
during the Middle Ages as marking the epoch of
transition from chaos to an ordered world. The
corresponding Byzantine date was 5509 B.C. The
whole of geological history had therefore to be
squeezed into about six or seven thousand years,
and this limitation naturally demanded some
extraordinary hypotheses to uphold it. As Prof.
Sollas says, " In the days when Geology was young,
it found a careful foster-mother in Theology, who
watched over its early growth with anxious
solicitude, and stored its receptive mind with the
most beautiful stories which the young science
never tired of transforming into curious fancies
of its own, which it usually styled ' Theories of the
Earth.' '

At the time of the revival of learning at the close
of the 1 5th century, men's ideas of the earth's
past history were largely dominated by the
exaggerated effects ascribed to the Noachian
Deluge. This devastating catastrophe was the type
of a succession of destructive cataclysms which
was believed to have preceded it. Supported by
Jewish cosmogony, and in harmony with the
scriptures, this view prevailed as the Doctrine
of Catastrophism until well into the igth cen-
tury. It was believed by some writers that all
sediments were deposited during the Flood ; others,
impressed by the succession of different types of
deposits found far from the present sea-level, and
often containing fossils, considered that one flood
was insufficient. As far back as the Qth century
Rhabanus Maurus, in explaining this phenomenon,
called to his aid three great universal floods, the

TIME PROBLEM AND ITS HISTORY

two later ones being contemporaneous with Jacob
and Moses respectively, but these floods apparently
were not as convincing as that of the time of Noah,
for they appear no more in geological literature.
Marine sedimentation of antediluvial times was
believed by some to have been extraordinarily
active, and this was added to the effects of the
Deluge to relieve the latter of the enormous amount
of work for which it was held responsible.

Fossils were regarded with suspicion and
jealousy, and most of the early naturalists
resolutely set themselves against the obvious
deduction to be drawn from them. About 300 B.C.,
Theophrastus had failed to see in them the
evidence of past life, and according to this philo-
sopher a " plastic virtue latent in the earth " was
supposed in some mysterious way to have given
them birth. The process was occasionally ex-
plained as being akin to crystallisation, but other
authors, gifted with more fascinating powers of
imagination though with less philosophic insight,
called to their aid the occult powers of " lapidifying
juice " and the obscure consequences of " tumul-
tuous movements of terrestrial exhalations." There
were even those who thought that fossils were the
work of the devil, subtly designed to draw
believers away from the faith.

Nevertheless, there were, from time to time,
more rational thinkers to whom such ideas were
both repugnant and untenable. Endowed with a
keener perception than their fellows, and with a
more critical temperament, they felt compelled
to regard fossils as organic remains. Leonardo
da Vinci (1452-1519) and Steno (1631-1686) were

THE AGE OF THE EARTH

amongst the most illustrious of these early
observers. The work of Steno, published in 1669,
shows how seriously he was handicapped by the
orthodox belief that only a few thousand years
had elapsed since the beginning of the world. Any
ideas which demanded longer periods were contrary
to revealed truth and were therefore bitterly
opposed. Steno demonstrated not only the true
nature of fossils, but also the orderly succession of
stratified deposits. He traced the structure of
the rocks to their subsequent movements, and
their surface features to the destructive agencies
of denudation. These remarkable investigations
distinguished Steno as an observer far in advance
of his age. The revelation of a tiny chapter of
the earth's past history which was thus afforded
him must surely have suggested the necessity of
calling more liberally upon time. However,
whether or not he recognised how hopelessly
inadequate was the period at his disposal, he
remained the victim of theological prejudice, and
cautiously avoided speculations as to the antiquity
of the earth.

Gradually, as the result of careful and patient
work like that of Steno, the foundation stones of
modern geology were laid in the face of a bitter
antagonism. Generelli in Italy in 1749, and Des-
marest in France in 1777 established the import-
ance of the slow but ever active processes at work
in the evolution of the earth's surface features.
It was found no longer legitimate to evoke forces
more intense, upheavals more violent, or catas-
trophes more devastating than those of present
experience.

TIME PROBLEM AND ITS HISTORY

Closely following these courageous authors, and
independently of their influence, came in 1785 the
Theory of the Earth of Hut ton. In this epoch-
making work the principle was defined which made
dynamical geology possible and which has proved
to be of the greatest assistance in wresting from the
rocks their history. It was asserted that the
activity of the agencies which had moulded the
earth's surface in the past and brought it to its
present condition still remained undiminished in
intensity. In existing causes lay the key with
which to unlock the secrets of the past.

The doctrine of Uniformitarianism, as it came to
be called, naturally implied that the earth's age
should be restricted no longer by dogmatic
obstinacy. To Hutton time scarcely presented
a difficulty. He found it at his unlimited disposal
both in past and future, and he concluded his
enquiries with the assurance that he found " no
vestige of a beginning — no prospect of an end."
He did not, however, infer that the world had
neither beginning nor end, a view for which some
of his critics held him responsible. On the contrary,
he carefully pointed out that in tracing back the
course of events we are at last limited in our
retrospect, and that beyond the dim horizon of
those early times stretches an unknown past.
Concerning this past, Hutton preserves an open
mind. It is not time which fails but data ; and
as he says " to reason without data is nothing but
delusion."

Hutton's convictions were regarded with
righteous horror by the official leaders of the day,
most of whom combined the study of theology

THE AGE OF THE EARTH

with that of their favourite science, and demanded
in the latter a harmonious agreement with the
scriptures. From their point of view Catastrophism
had the advantage, and they were firmly persuaded
of its truth. Fifty years had yet to elapse before
the superiority of many of Hutton's opinions came
to be generally recognised, and even then the
fallacies underlying the earlier doctrines were
but grudgingly admitted.

The opening years of the igth century were
signalised by the work of Lamarck and Cuvier in
palaeontology. Lamarck recognised that fossil shells
were most commonly of marine types and that in
the gently accumulating sediments of the sea-
bottom they were buried and preserved. In the
succession of faunas which he studied, and in
his belief that by some law of development they
were all related, he found a cogent argument in
favour of the great antiquity of the earth. Cuvier,
however, was led by his researches to extend the
conception of great world convulsions. Time after
time the earth's inhabitants had been destroyed
and entombed, only to be replaced by the creation
of fresh types after the force of the cataclysm had
subsided. The Deluge was the last of these up-
heavals, and a scientific proof of its widespread
effects was drawn from the superficial deposits
found in so many parts of the globe.

Until the barren ideas of Catastrophism had been
abandoned, the past provided little more than an
exercise for the imagination. But the immature
days of geology were passing away. The solid
work of William Smith prepared the way for an
accurate historical development of the science.
8

TIME PROBLEM AND ITS HISTORY

Scrope drew attention to the importance of
studying geological processes actually in operation,
and arrived at conclusions essentially the same
as those of Hutton. Finally, during the years
1830-1833, the first edition of Ly ell's Principles
was issued, a work which set flowing the full
tide of Uniformitarianism. Ten years previously
Lyell had felt compelled to renounce the unsound
doctrines of his teachers, and with the publication
of these carefully reasoned volumes he became the
champion of the rival position. He denied the
former existence of catastrophes of an order of
magnitude different from those of the present.
In their place he demanded only time. A short
but convulsive past was to be exchanged for a
longer one, uniform and comparatively tranquil
throughout.

Lyell's views did not find many ready adherents
among the older geologists. Buckland at Oxford
and Sedgwick at Cambridge had long taught a
system of geology which they believed to be in
accordance with the first few chapters of Genesis.
Eventually, however, they found that under more
careful examination the evidence of a universal
flood faded away, and with them and their
generation the tendency to demand a literal agree-
ment between geological theory and the writings of
Moses gradually disappeared. A new fraternity of
geologists arose whose ideas were limited by fewer
prejudices, and who found the time barriers raised
against them no longer.

Geology was now able to stand firm on its
own evidence, and to insist without fear of con-
tradiction that long periods must have elapsed

THE AGE OF THE EARTH

during the slow accumulation of strata. The
burden of reconcilation now fell upon the theo-
logians. Happily, the days of malice and persecu-
tion had gone by, and in the light of a more broad-
minded criticism it was found that the Church
had nothing to lose by the rejection of Bishop
Ussher's chronology. The first three words of
the Bible, " In the beginning," were interpreted
afresh, and from the indefinite lapse of time which
they seemed to imply the geologist was allowed
to draw at will.

From this period until 1862, when Thomson
(Lord Kelvin) attacked the problem in an entirely
new way, there was no necessity to apologise for
the most extensive drafts on the bank of time,
and no further restraint was felt in estimating
the antiquity of the earth. But the data were
still inadequate, and many mistakes were made.
Most of the estimates put forward were afterwards
regarded as wildly extravagant, and naturally,
they have no intrinsic value to-day. The stratified
rocks, representing the aggregate accumulation of
material denuded from the lands, afforded a
valuable time-index. It was recognised that the
processes of denudation and deposition, like a
gigantic hour-glass, had been recording time since
land and sea were first defined. But the record was
perplexingly difficult to read, and the time units,
based on estimates of present rates of erosion
and sedimentation, were little more than guesses,
uncertain and even misleading.

The only interpretation worthy of serious
consideration was that of John Phillips, who
concluded in 1860 that the time required for the

TIME PROBLEM AND ITS HISTORY

deposition of the complete succession of strata lay
between 38 and 96 million years. As an example
of one of the more extravagant demands, mention
may be made of Darwin's estimate in 1859 °* the
time elapsed since the latter part of the Cretaceous
period. From the rate of chalk erosion in Kent,
he deduced that the excavation of the Wealden
valleys had required a period of 300 million years.
Referring to this estimate, Jukes gave his opinion
that while it might be a hundred times too great,
it was equally probable that it was a hundred times 1
too small. Evidently 30,000 million years, in the \
absence of more reliable data, was not considered
an absurdly excessive period for the erosion of the
Weald. It should be noticed, however, that after
the publication of Thomson's views, Darwin with-
drew this estimate from the Origin of Species.

We must now return to consider the problem
as it appeared to the leading physicists of half a
century ago. In 1842 Mayer, and during the
following year Joule, had placed the conception of
the conservation of energy on a firm experimental
basis, and started the principles of thermo-
dynamics on their brilliant career through the
sciences. Twenty years later, Thomson invaded
the domain of Geology, hoping to reform its
speculations and bring them into accordance with
the doctrines of the conservation and degrada-
tion of energy, doctrines which were apparently
set in defiance by the orthodox tenets of the
science.

As early as 1852 he had shown that under the
laws to which matter and energy are subject, the
earth within a finite period of past time must have
ii

THE AGE OF THE EARTH

been unfit as a habitation for life. This general
conclusion he afterwards worked out in detail,
determined to protest against what he considered
the immoderate application of the principle of Uni-
formity. So profoundly did he influence geological
opinion, that for nearly fifty years the question of
the earth's antiquity centred around his name.
A short account of his contributions to the physical
aspect of the subject, and of the famous contro-
versy which they aroused, cannot therefore be
dispensed with.

In 1855 Thomson indicated the way in which
observations of underground temperatures might
be applied to the determination of absolute dates
in geological history. Already the origin and
maintenance of the sun's heat had been discussed
by Mayer and Helmholtz, and during the years
1860 and 1861 Thomson dealt with their respective
suggestions. During the following year, 1862,
came the epoch-making papers On the Age of the
Sun's Heat, and On the Secular Cooling of the
Earth.

The age of the sun's heat evidently restricts
geological time in a very decided way, and Thom-
son drew attention to the necessity of the fact that
unless the sun were a perpetual miracle, it could
not have continued to radiate heat-energy for an
unlimited period. His guarded conclusion was
that the sun most probably has not illuminated
the earth for 100 million years, and almost
certainly not for 500 million years.

Regarding the earth as a globe which had
gradually cooled down, he tried to show that the
principles of thermo-dynamics had been dis-

TIME PROBLEM AND ITS HISTORY

regarded by geologists. From the earth's present
store of heat, as revealed by underground tempera-
ture gradients, he calculated that the consolidation
of the crust took place about 100 million years ago.
Owing to the uncertainty of much of the data on
which this estimate was based, he allowed wide
limits. Had a solid crust formed permanently
less than 20 million years ago, underground heat
should be greater than is actually observed. Had
it formed at a date more remote than 400 million
years ago, then the temperature gradient near the
surface should have been notably less than it is.

In 1865 appeared a remarkable little paper in
which attention was drawn to the earth's thermal
history, and particularly to its more energetic
youth, the object again being to refute the doctrine
of Uniformity. During the next year Thomson
delivered the Rede Lecture on The Dissipation
of Energy, and showed the importance of the
tides in terrestrial dynamics. All over the ocean
the effect of the friction set up by moving water is
either to accelerate or to retard the earth's rota-
tion. The final result was, he asserted, to retard
the earth as though a gigantic friction brake were
being slowly applied. The ultimate tendency was
towards a state when relative motion between
earth and moon should be no more.

In 1868, in an address on Geological Time,
Thomson gathered together his three limiting
criteria, and further developed the application of
tidal friction to the question. In his discussion the
earth's figure was supposed to be a survival from
the period of consolidation ; a permanent record
of terrestrial conditions at that time. This being

THE AGE OF THE EARTH

so, the rate of rotation could be computed under
which that form would be in fluid equilibrium.

The three maximum time limits which Thomson
drew from his arguments were respectively 500,
400, and 1000 million years, with a lower limit of
20 million years. The final conclusion was " that
the existing state of things on the earth . . .
must be limited within some such period of past
time as one hundred million years."

The reply to this series of attacks on what was
supposed to be orthodox geological opinion, came
from Huxley in 1869. He did not deny the
consequences of the dissipation of energy in the
cases to which attention had been directed, but
decided in favour of the geological evidence,
which indicates, as time has more and more sub-
stantiated, that " they have made no practical
difference to the earth during the period of which
a record is preserved in the stratified rocks."

Thomson returned to the attack a month or
two later, protesting once more against this
attitude. At the close of his address, he said :
" A large proportion of British popular geologists
of the present day have been longer contented
than other scientific men to look upon the sun as
Fontenelle's roses looked upon their gardener.
' Our gardener,' say they, ' must be a very old
man ; within the memory of roses he is the same
as he has always been ; it is impossible he can
ever die, or be other than he is.' '

There were no further contributions to the
problem until 1876, when Thomson revised the
former conclusion which he had based on the
distribution of underground heat, and narrowed his

TIME PROBLEM AND ITS HISTORY

limits to 50 and 90 million years. In later years
he reduced both these limits still further, though
he was always more prudent than Tait, who with
little justification cut down the time allowance in
a most alarming way. Tait wrote in 1875, " Ten
million years is about the utmost that can be
allowed from the physical point of view for all
the changes that have taken place on the earth's
surface since vegetable life of the lowest known
form was capable of existing there." Assertions
such as this were among the most embarrassing
circumstances that geologists had to face. The
late Sir George Darwin was more kindly disposed,
although his contributions to the question were
considered to support the physical arguments.
Believing that the moon had been derived from
the earth by the separation of an enormous tidal-
wave, he calculated that since this stupendous
event at least 56 million years must have passed.
The physical evidence appeared at first to be
irrefutable, and the estimates based upon it
equally certain. Yet earth history could not
comfortably be squeezed into less than 100
million years. The stratified rocks were there
in undoubted succession ; mile after mile of
thickness with no indication of more rapid ac-
cumulation than that of modern deposits. In
spite of this, however, a prejudice in favour of
short estimates was gradually aroused, and some
attempt was made to hurry up geological activities
in the past in renunciation of the principles of
Uniformitarianism. But many geologists refused
to give way and vigorously attacked the physical
stronghold, searching out and exposing all the

THE AGE OF THE EARTH

assumptions, and noting with satisfaction the
uncertainty of much of the data and its doubtful
applicability. A few physicists themselves denied
that the limited estimates were securely founded.

In 1895 the controversy was re-opened by the
publication in Nature of a correspondence initiated
by Prof. Perry. While Perry had previously
accepted Kelvin's conclusions, he now challenged
the validity of all three. The strongest criticism
was directed against Kelvin's assumption of a
homogeneous earth. It was shown that if the
rocks of the interior, under high temperatures and
pressures, were to conduct heat ten times as well
as those near thesurf ace, then Kelvin's figures would
need to be increased fifty-six times. To settle this
point adequate data were not then forthcoming,
but Perry stated his belief that, if geologists had
sound reasons for demanding long periods of time,
he saw nothing which denied them four times the
greatest (1000 million years) of Kelvin's estimates.

Kelvin wrote that he would rather know the date
of the consistentior status than that of the Norman
Conquest, so interesting did he find the subject.
After remarking that 100 million years were
ample to satisfy Geikie, he said, " I should be
exceedingly frightened to meet him now with only
20 million in my mouth." However, Kelvin
carefully examined the data referring to the
conductivities of rocks and decided that they were
not favourable to Perry's suggestion. Thus
fortified, he lent his support to the independent
estimate of 24 million years which had been arrived
at in 1893 by Clarence King.

Kelvin's last pronouncement of his views was

TIME PROBLEM AND ITS HISTORY

in 1897, when he delivered an address on The
Age of the Earth as an Abode Fitted for Life.
He then narrowed down his earlier estimates to 20
and 40 million years. To most geologists these
limits were seriously in conflict with the require-
ments of their science. A feeble attempt was
made to force an agreement, but it was generally
held that the sedimentary succession implied a
period three times as long. An independent mode
of reckoning geological time was developed by Prof.
Joly in 1899 He made a careful study of the amount
of sodium annually removed from the land by
solvent denudation, and calculated the time during
which the total quantity of sodium now held by
the oceans could have accumulated. The period
closely approached 100 million years, and thus
further support was added to the testimony of the
rocks, for with the birth of the ocean their
deposition must have commenced.

Among many of the more optimistic geologists
there was a prevalent expectation that some flaw
would ultimately be found in the physical argu-
ments. Their anticipation was realised ten years
ago in the most surprising and unexpected way.
In 1903 came the discovery by Curie and Laborde
that radium maintains a temperature above that
of its environment owing to the spontaneous
evolution of heat involved in its disintegration.
Other investigators found that radium and its
radioactive associates were widely distributed in
the earth's surface materials. In all waters and
gases of natural origin, and all rocks and soils,
traces of these elements have been detected. We
owe a great deal to the careful researches of Prof.

c 17

THE AGE OF THE EARTH

Strutt, which have shown conclusively that the
earth can no longer be regarded merely as a cool-
ing body. A newly recognised source of heat
must now be taken into account, and indeed, so
relatively abundant is the supply, that our present
difficulty is to understand why the earth is not
hotter tnan we actually find it.

With these discoveries the long controversy
was finally buried, and Kelvin's treatment of the
problem was proved to have been fallacious.
Kelvin lived just long enough to know something
of the first attempts to utilise the principles of
radioactivity in solving the vexed problem of
time. The discovery of radium did not only
destroy the validity of the older thermal argu-
ments ; but also, it led directly to the elaboration of a
new and more refined method. As we shall see in the
sequel, every radioactive mineral can be regarded
as a chronometer registering its own age with
exquisite accuracy. The record is not always
completely preserved, but a few attempts have
been made to read it, and in the more favourable
cases, periods of enormous duration have been
revealed. Indeed, if our interpretation is correct,
some of the oldest Archean rocks must date back
1600 million years.

Not many years ago geologists were dissatisfied
with the shortness of their time allowance ; to-day
they are confronted with an embarrassing super-
abundance. Certainly, it has been dogmatically
denied that radioactive minerals can have the
great ages which have been attributed to them,
or that they can account for more than a small
proportion of the earth's loss of heat. But such
18

TIME PROBLEM AND ITS HISTORY

denials do not help to remove the difficulty ; they
rather tend to aggravate it. In the face of two rival
and mutually inconsistent systems of earth
chronology, neither of which can be rejected with
impunity, it becomes essential to examine most
carefully the fundamental assumptions underlying
each method, in the hope of detecting the subtle
errors which are responsible for so glaring a
disagreement. In this way alone can a means of
reform be indicated and the road made clear for the
construction of a time-scale which will be accept-
able to all.

It is obvious that as yet we cannot measure
the earth's absolute age if by that expression is
meant the time which has elapsed since our
planet first existed. Kelvin's work most nearly
approached this desideratum, but, as we have
seen, it was doomed to ultimate failure, though
not indeed, to discredit. Sir George Darwin's
calculation of the lapse of time since the
birth of the moon stands apart ; his results are
limited only by a definite minimum, and otherwise
are sufficiently elastic to meet any reasonable
demand. But this convenience, added to the
doubtful validity of the hypothesis on which his
estimates were based, detracts from their value.
It would be unfair to expect too much of them,
for they were only put forward in support of
a hypothesis which would have been immediately
disproved if ridiculously long or short time periods
had been involved.

As we shall see in the following chapter we are
still far from understanding the sequence of events
which led up to the origin of the earth. All we

THE AGE OF THE EARTH

can hope to do is to fix the dates of critical
periods of its history and assign its origin to
a point still more remote. The different methods
which have been elaborated to deal with the
problem are all based on a common principle.
The rates of certain changes at the present
day are determined as accurately as possible,
and in imagination, the respective processes
are traced backward in time, until limiting
conditions are arrived at. Thus, Kelvin takes
us back to a time when the earth was not
yet a solid globe ; Darwin traces back the moon's
history until he finds it revolving close to the
earth ; Joly bids us imagine the oceans in their
original freshness, free, or nearly so, from salt;
Geikie finds an end at last to the long suc-
cession of stratified rocks and seeks to estimate
the time they represent. Last of all, and most
brimful of promise, there lies in the mechanism
of radioactivity an elegant method for assigning
a date to the period of crystallisation of every
igneous rock in which suitable minerals can be
found.

The testimony of biology scarcely calls for
consideration here, for as Huxley pointed out,
biology must take its time-scale from geology.
The procession of life forms shows broadly that the
time involved must have been very great; and,
moreover, where we first meet it in the Cambrian
rocks, it is already far on its journey. This is
usually taken to signify that pre-Cambrian time
must be at least as long as that which has elapsed
since. Such a statement can only be regarded as
very conservative.

TIME PROBLEM AND ITS HISTORY

Prof. Poulton considers that the process of
evolution must have required much longer
periods of time than those estimated by the
geologist. Sollas, on the contrary, is satisfied
that 26 millions of years would be ample to
meet all the demands of biology. It is obvious
that a numerical estimate cannot be derived
from the succession of organic forms, for the
rate of change of species is as yet known only
relatively. The influence of changing environment
is complex beyond the possibility of exact analysis,
and any method of chronology based on the
development and ultimate extinction of past
types would necessarily give results of very
unequal value. The little shell Lingula has
retained its individualit}/ with but little deviation
from its original form throughout the time repre-
sented by the fossiliferous strata, but the same
record is eloquent of the gradual unfolding of
fauna after fauna, culminating at last in the
highly specialised organisms of to-day.

CHAPTER II
TIME AND TIDE IN THE SOLAR SYSTEM

Early ideas of cosmogony — The Laplacian hypothesis —
Tidal friction in the earth-moon system — Its application
by Kelvin to the question of the earth's antiquity — Its
application by Sir G. Darwin to the history of the moon
and the date of its birth — Difficulties in the way of the
Laplacian hypothesis — The Planetesimal hypothesis — The
origin of the ancestral solar nebula and its transformation
into the solar system — Early stages of the earth's history.

OPECULATIVE fancies concerning the origin
»^ of the world form the subject-matter of many
of the earliest writings on record, and throughout
the intellectual history of mankind the problem
has proved to be one of supreme fascination. It
was not, however, until quite recent times that the
efforts of imagination gave place to reasoned
hypotheses, tempered by a more sober regard for
physical probabilities. At first, on having attained
the status of a science, geology steadfastly refused
to consider seriously the cosmogonic fantasies
then current. It was Hutton, who by advocating
the direct observation of nature in place of the old
scholastic arguments, first delivered geology from
the inevitable wranglings that would necessarily
have arisen from so premature a discussion of the
beginning of things. Cosmogony, in spite of this,
continued to receive attention from workers in
other sciences, and while to-day we are still unable

TIME AND TIDE IN SOLAR SYSTEM

from geological facts alone to trace back with con-
fidence the details of the earth's beginning, yet
the uncertainty which justified Hutton in entirely
disregarding the earth's genesis no longer exists.
Astronomy, physics, and chemistry have all con-
tributed to the elucidation of what may be called
the prehistoric period, and have done much to
remove our modern ideas from the dangerous
quicksands of speculation.

It is becoming more and more evident that many
of the fundamental problems of geology can be
solved only with reference to the processes involved
in the making of the earth and in its subsequent
evolution. The dynamic agencies at work to-day
are' genetically the outcome of the ancestral forces
which first moulded our planet, and theoretically,
the tectonics and constitution of the earth's crust
should lead back to a more complete understanding
of its initial condition. But the mind of man is im-
patient of delay ; shrewd guesses are made and
gradually adjusted to known facts, with the result
that many geological doctrines are founded not
on observation alone, but also in part on funda-
mental hypotheses.

During the igth century scientific thought was
powerfully influenced by the achievement of
Laplace, for it was he who first presented the
famous Nebular hypothesis in a consistent and
acceptable form. Previous thinkers, notably
Buffon in 1745, and Kant in 1755, had put forward
similar generalisations, but they fell into certain
errors which Laplace escaped, and their exposi-
tions lacked the completeness and authority
which his mathematical genius guaranteed. In the

THE AGE OF THE EARTH

Sy si erne du Monde, published in 1796, Laplace
attempted to trace the gradual evolution of the
solar system from a spheroidal nebula, under the
normal operation of natural laws. He started with
a hot gaseous nebula of lens-like shape extending
beyond the present orbit of the outermost planet
and rotating slowly in the same direction as that of
the sun. The average density of such a nebula would
be about 1/250,000,000 that of ordinary air. As
Laplace explicitly states that the sun was already
foreshadowed in the nebula as a strongly condensed
central nucleus, the outer atmosphere must have
been of inconceivable tenuity, a condition main-
tained only by intense heat. Through the loss of
this heat by radiation, and under the influence of
its own gravitation, the nebula would slowly con-
tract. In accordance with the laws of dynamics,
contraction would necessarily be accompanied by
a compensating increase in the rate of rotation.
At a certain stage, the centrifugal force at the
Equator would balance the attraction of gravity,
and a ring of gas would be left behind. The same
process of ring separation would afterwards be
repeated from time to time as contraction pro-
gressed, and each ring being inherently unstable,
would rupture, ultimately forming a spheroidal
mass with the requisite directions of rotation
and revolution. The Asteroids, a group of tiny
planets revolving in the zone between Mars and
Jupiter, probably represent a ring which failed to
coalesce into a single planet. In their newly-
born condition the planets were still hot and
gaseous, and by the operation of the same
mechanical agencies on a smaller scale most of
24

TIME AND TIDE IN SOLAR SYSTEM

them detached a second generation of rings, and
these having aggregated in turn became the
satellites. The great central nucleus, continuing
to contract after the last planetary ring had been
abandoned, became the sun.

On this hypothesis the earth was originally a
hot fluid globe with a heavy atmosphere consisting
of the vapours of its more volatile constituents.
As it contracted it rotated faster and faster, until
a limiting velocity was attained which allowed the
separation of the ring which ultimately condensed
to form the moon. At a later stage the earth began
to solidify and the crust first made its appearance.
The lighter igneous rocks, the granites and
corresponding lavas, presumably formed the outer
layers, while beneath this zone the more basic
magmas arranged themselves in the order of their
density. When cooling had sufficiently progressed,
the waters condensed to form the oceans, and
with the emergence of the lands, denudation began
for the first time and the earth's historical period
was inaugurated.

At the time of the birth of the moon the rotatory
velocity of the earth must have been very rapid
compared with that of the present day, and an
obvious deduction is that the earth has gradually
slowed down in the course of its history — in spite
of the accelerative effect of contraction — and is
probably still being retarded. From a comparative
study of ancient and modern eclipses, Dunthorne
demonstrated in 1749 that the earth appeared to be
losing time. More than a century later, Adams
showed from a revision of all the data, that the
loss amounted to 22 seconds in a century, although

THE AGE OF THE EARTH

he was careful to point out that the assumptions on
which his calculation was based were themselves
not securely founded . It was Kant who started the
idea that the ceaseless operation of tidal friction
would tend to bring about this result. Long
afterwards Kelvin indicated the way in which
tidal retardation might be applied to estimate the
date of the earth's solidification. He supposed
that the form then assumed by the earth had
survived in its essential features throughout
geological history. Subsequent diminution of the
rate of rotation was not considered to be attended
by a corresponding change of oblateness. Grant-
ing this assumption, the present figure of the earth
ought to inform us of the rate of rotation under
which it would be in fluid equilibrium at the time
of consolidation. After making these concessions
to speculation, Kelvin safeguarded himself by
assigning wide limits to the earth's age. His
maximum estimate was a liberal 1000 million years,
but he further stated his opinion that if a much
higher antiquity than 100 million years were
demanded, the polar flattening and equatorial
bulging should be more marked than we find
them.

These conclusions were necessarily somewhat
vague, and based as they were on a very question-
able assumption, they were readily susceptible
to damaging criticism. It is known from the
concordant evidence afforded by seismic, geo-
physical, and astronomical phenomena that the
earth is essentially solid throughout with a
rigidity more than twice that of steel. So far our
conception of the earth is similar to Kelvin's, but

TIME AND TIDE IN SOLAR SYSTEM

the postulate that such a body could resist rota-
tional changes without modification in shape, can-
not now be granted. As a whole the earth tends to
conform to the laws of fluids, though its response
may be tardy and the alteration of form may lag
considerably behind the ultimate cause. This
being so, it would be more correct to refer Kelvin's
time limits to the period of " lag " rather than to
the period of the earth's history as a solid globe.

Sir G. Darwin's departure from the ring con-
ception of Laplace in the case of the moon, and his
alternative theory of the moon's origin and history
are well known. Tracing back to their logical
conclusions the effects of tidal friction in the earth-
moon system, he has developed one of the most
fascinating romances in the domain of cosmogony.
Unfortunately it is impossible to apply any definite
dates to the critical stages of this history, if for no
other reason than the imperfection of our present
lunar and tidal theories. His lower estimate of the
time which has elapsed since the disruptive catas-
trophe lies between 50 and 60 million years, but
the actual time would certainly be much greater.
Indeed, Darwin himself, referring to the problem
of cosmical time after the advent of radium and
the overthrow of the older standards, wrote, " I
feel with some degree of confidence that if the
tidal theory shall ultimately be condemned it will
not meet its execution on the score of lack of
time." A recent discussion of the problem by
Prof. Chamberlin and his colleagues is unfavourable
to Darwin's view. After a careful study of the rate '
of tidal retardation they show that the lengthening
of the day is of the order of one second in 500,000
27

THE AGE OF THE EARTH

years, and that if the physical condition of the
earth has remained essentially as at present, at
least 220,000 million years must have elapsed since
the day occupied 20 of our present hours and the
month 29 such days. Yet just after the birth of
the moon, as outlined by Darv/in, day and month
were alike in duration, each being equal to about
three of our present hours. It must be confessed
that, after all, we know but little of the moon's
history with certainty.

At the time of its announcement to the world
the Laplacian theory was remarkably in accordance
with the knowledge of the time. But as further
discoveries were made, numerous inconsistencies
and contradictions arose. Not only were certain
of the movements of the planets and satellites
found to be in a retrograde direction, but, besides
these awkward facts, at every stage of the postu-
lated development of the solar system insuperable
mechanical difficulties appear which render the
truth of the hypothesis highly improbable. Finally,
in the dynamics of the system there are so
many discrepancies between the requirements of
theory and the actual circumstances that Prof.
Moulton and Dr. See both insist that it could not
have originated in the way that Laplace imagined.
The heavens have been diligently searched for
nebulae of the Laplacian type, but no certain case
has been observed. The vast majority conform to
the spiral type and to a nebula of this kind appeal
has been made in the recently developed Planet-
esimal hypothesis of Chamberlin and Moulton.

The general structure of a spiral nebula is
immediately suggestive of tidal action. From a

FIG. 5.
Symmetrical Nebula in Piscium, M 74.

FIG. 6.
Whirlpool Nebula in Canes Venatici, M 51.

SPIRAL NEBULA.

TIME AND TIDE IN SOLAR SYSTEM

central nucleus two spiral arms emerge at dia-
metrically opposite points. Often they are
beautifully defined (Figs. 5, 6), but seen from the
side they appear as discs of misty light. In the
arms nebulous knots and irregularities are generally
apparent, the precursors, perhaps, of a system of
bodies analogous in their distribution to the
planets. To account for the existence of a spiral
form, conditions of extreme tidal distortion are
suggested. In the case of our ancestral sun such
conditions would be brought about by the close
approach of another celestial body. As the stranger
drew near, the tidal stresses set up would gradually
increase until explosive outbursts of matter from
the sun were projected in the plane of attraction,
one on the near side, another on the far side. The
result of the combined attractions of the two suns
on the ejected material would be to develop a
spiral structure. An enormous number of tiny
planets or planetesimals would thus begin to
circulate about the sun, associated with scattered
knots of larger dimensions which would form the
nuclei of the future planets and satellites. These
knots would at once begin to grow by coalescence
at the expense of the vast numbers of planetesimals
associated with them in adjacent zones. Dynami-
cally the scheme is sound, and the many suggestive
consequences which unfold themselves explain away
most of the difficulties which proved so embarrass-
ing to the Laplacian hypothesis. Into a discussion
of these advantages space does not allow us to
enter, but it may be said that of all the attempts
to grapple with the fundamental problem of the
genesis of the solar system, the Planetesimal
29

THE AGE OF THE EARTH

hypothesis appears to be the most successful. It
is to be welcomed, apart from its many convincing
features, if only because of its stimulation to the
further study of the early stages of earth-history.

The earth began on this hypothesis as a nebular
knot, and it has since grown up to its present mass
by the capture of outside planetesimals. It is very
unlikely that it was ever in a molten condition.
Internal heat arose in part from the condensation
of the mass during the period of its growth. The
temperature would slowly rise until the fusion
point of certain of the constituents was reached
and the liquid tongues and pockets thus formed
would then tend to move away from the centre —
the lighter and less viscous stony material being
squeezed upwards relatively to a network of the
heavier and more rigid metallic material. Once
vulcanism had been initiated in this way the pro-
cess would continue until a highly metallic nucleus
had collected. Surrounding it there would gradually
form a thick zone of silicate rocks, the differentia-
tion from the original heterogeneous mixture of
stony and metallic constituents being due to the
selective fusion of the former. In dealing with the
question of the earth's heat in a later chapter, the
value, and indeed the necessity of this conception
will be realised.

The atmosphere and oceans must have been
derived from the planetesimals themselves, and on
analogy with rocks and meteorites, the planetesi-
mals would not be lacking in the raw material from
which to evolve them. Even now, the outer 70 miles
of the earth's crust would be competent to supply
all the nitrogen of the atmosphere, the water of the

TIME AND TIDE IN SOLAR SYSTEM

oceans and the vast quantity of carbon-dioxide
represented by limestones and carbonaceous
deposits. With the existence of an ocean and
atmosphere, a new factor in surface differentiation
arose. Mechanical and chemical denudation
became possible, and the first sediments were
deposited. Although the earth's growth had not
yet ceased, all the agents occupied in its subsequent
development were now at work and its geological
history may be said to have definitely commenced.

CHAPTER III

ASTRONOMICAL CONSIDERATIONS

The Great Ice Age — Extension of glaciation — Croll's
ingenious hypothesis — Inadequacy of the explanation —
The ice caps of Mars — De Geer's discovery of the annual
layers of glacial clay in Sweden — Application to the mea-
surement of time — Sederholm's observations in Finland —
Cyclic sedimentation in the Cretaceous strata of Colorado
— Its correlation by Gilbert with an astronomical time-
unit.

IN the last chapter mention was made of
Kelvin's work on tidal friction and of Sir G.
Darwin's speculations as to the early history of
the earth-moon system. Their conclusions de-
pended very largely on a cosmogony which has
failed to meet with general acceptance. A very
different appeal to astronomical causes was made
by Croll in his famous attempt to account for the
anomalous conditions of the glacial period. Here
we may also consider the correlation by Baron De
Geer and Mr. G. K. Gilbert of certain unusual
phases of sedimentation with the seasonal and
climatic changes brought about respectively by
the earth's motion and its periodic fluctuations.
All of these investigations find a common basis in
their direct reference to astronomical considera-
tions.

In the course of its history the earth has
undoubtedly passed through several periods of

ASTRONOMICAL CONSIDERATIONS

extreme cold, periods when Arctic conditions
swept down from their polar strongholds and
invaded the temperate zones and even the tropics.
Ice and snow have left their traces in many a
grooved and striated surface, and in gently rounded
outlines the landscape often betrays their former
presence. Erratics and perched blocks, terminal
and lateral moraines, lakes and alluvial terraces,
U-shaped and hanging valleys all bear witness to
the wide extension of the vanished glaciers of the
past. Relics of glaciation have been preserved in
the Cambrian or pre-Cambrian rocks of Norway,
China, Australia and South Africa. Still more
remarkable are the records of a Permian ice age
found in the rocks of South America, South Africa,
India and Australia. No unequivocal evidence of
glaciation in later periods is forthcoming until the
advent of the Pleistocene. The Great Ice Age
through which the earth has so recently passed
was not, however, a single epoch of glacial exten-
sion. Four episodes in the climatic cycle can be
recognised — a genial episode, a period of falling
temperature and glacial advance, a glacial episode,
and finally, a period of rising temperature and
glacial retreat. That this cycle has been four
times repeated is the testimony of the Eastern
Alps as interpreted by Prof. Penck. In the British
area the ebb and flow of temperature has not left
so clear a record, and a more continuous and per-
sistent glaciation appears to be indicated.

At the time of the maximum advance of the ice,
all northern Europe lay buried beneath an immense
ice sheet, which was fed by enormous glaciers
slowly creeping down from the uplands. Curiously

D 33

THE AGE OF THE EARTH

enough, Siberia, which in parts experiences the
coldest winter in the world, is not now and was not
then covered by the ice-field. Farther south the
ice collected in the Pyrenees, Alps, Caucasus and
in the great Asiatic ranges and descended in all
directions far below the level of the present snow-
line. In Canada and the United States the ice
mantle proceeded from three great centres — from
Labrador along the Atlantic coast, from the Keewat-
in district by Hudson's Bay and from the Cordilleras
along the Pacific coast. In the tropics and in the
southern hemisphere the story is the same. Down
the slopes of Kenia, Kilima N'jaro and Ruwenzori,
the towering peaks of Central East Africa, the
glaciers descended 5000 feet below their present
termination. Kosciusko in New South Wales bears
conspicuous traces of a former ice-cap which
completely shrouded all but the highest peaks of
the plateau. From the highlands of Tasmania
and New Zealand, and from the Cordilleras and
Andes of Patagonia and Chili the evidences of
a prolonged glaciation are equally clear.

No satisfactory theory of climate has yet been
propounded which affords an adequate explanation
of this universal lowering of the snow-line.
Whether or not the temperature fluctuations were
synchronous all over the earth is difficult to decide,
but that a colder climate characterised the southern
no less than the northern hemisphere cannot be
doubted. The causes to which appeal has been
made in accounting for climatic changes are of three
classes, — astronomical, geographical and atmo-
spheric. In general the various tendencies will be
opposed to one another and by their interference

ASTRONOMICAL CONSIDERATIONS

prevent the attainment of universal extremities of
climate. Occasionally, however, circumstances
may arise when their joint action will lead in a
single direction. A wide extension of tropical or
polar conditions would then be expected to follow.

Of the three contributory causes, only one, the
astronomical, bears any de terminable relation with
time. The theory that secular variation of
terrestrial climates results from the changing
eccentricity of the earth's orbit, was first proposed
by Adhemar. In 1868 James Croll greatly
elaborated and extended this theory, presenting
it with such a formidable array of quantitative
data and yet in such an attractive form that it
exerted a considerable influence on the geological
thought of the day. Not only did it seem to offer
an acceptable means of escape from the maze of
difficulties by which the problem was shrouded,
but in addition it promised a faithful chronology
fixing the date and duration of the Ice Age with
almost ideal precision.

Laplace established the fact that the ellipticity
of the earth's orbit is subject to periodic oscilla-
tions between certain limits. Sometimes the
path is nearly circular but at other times it becomes
a more flattened ellipse. If the earth were the only
planet, its orbit would suffer no change ; that it
does is due to the attractions of the sister planets.
Nevertheless, the orbit is essentially stable, and
the yearly journey always occupies the same time.
Formulae were devised by Leverrier from which
it was possible to calculate with some accuracy,
the actual value of the eccentricity at any given
period in past or future within a few million years

THE AGE OF THE EARTH

of the present. Croll utilised these formulae to
compute the dates of maximum and minimum
eccentricity for the past three million years. He
found three important periods when that factor
was specially high, betraying a type of orbit more
than usually flattened. These were :

(a) from 2,500,000 to 2,600,000 years ago.

(b) ,, 720,000 to 980,000 ,, ,,

It was to the last of these that the Glacial Period
was assigned.

When the earth is at perihelion, i.e. in that part
of its orbit which lies nearest to the sun, it enjoys
a more generous radiation than falls upon it at
aphelion, when it is farthest from the sun. But
this alone does not determine the time of summer
and winter. At present the northern winter and
southern summer occur when the earth is in
perihelion. As is well known, this is owing to the
inclination of the earth's axis and the blanketing
effect of the atmosphere. In the northern hemi-
sphere, the greater thickness of air presented to the
sun's rays during the winter keeps out more heat
than is gained by the relative closeness to the sun.
However, this condition is not permanent. As
discovered by Hipparchus in the year 134 B.C., the
positions of summer and winter and of the equi-
noxes on the ecliptic are subject to a slow forward
movement. In 26,000 years they make the com-
plete circuit, and so in the course of time the
relation of the seasons to perihelion is slowly
altered.

The precession of the equinoxes was shown

ASTRONOMICAL CONSIDERATIONS

by Newton to be a dynamical consequence
of the spinning of the earth about a tilted
axis. While this change is progressing, the
position of perihelion is also shifting, and the
resultant period is thereby reduced on the average
to 21,000 years. Thus, in about 10,000 years from
now the northern hemisphere will enjoy summer
at perihelion. At the same time, the northern
winter will occur at aphelion. The cold season
will then be longer and more severe than now,
and the annual accumulation of snow correspond-
ingly increased. If, in addition, we suppose the
orbital eccentricity to approach its maximum
value, the northern hemisphere would then be in
the grip of winter for nearly four months of the
year, and the cold would become still more bitter
than before. As a slight compensation for the
rigours of the winter, the summer, though short,
would be very much hotter. It was extremes
such as these, accompanied perhaps by favourable
geographical conditions, that Croll postulated for
his glacial period. He believed that the snow and
ice which would collect during a long frosty
winter would successfully resist the evaporative
powers of the summer, and that permanent snow-
fields would therefore arise. Once started, the
snowy mantle would tend to continue. " A great
deal of heat could be absorbed without raising the
temperature above freezing-point, and the result
of evaporation would be the creation of a thick
blanket of fog, an effective agent in guarding the
ice against the ravages of the summer radiance.

A peculiarity of Croll's hypothesis is that
glacial epochs could not exist in both hemi-

THE AGE OF THE EARTH

spheres at the same time. South of the
Equator the summer would be longer than it is at
present, and the winter would not only be short,
but also comparatively mild. The climate would
be that of a genial interglacial period. The
alternation of glacial epochs between north and
south is a necessary consequence of Croll's view,
but when the facts are examined they are found to
be unfavourable to this assumption. In Sweden
the last period of extension of the ice appears to
have been synchronous with that in New South
Wales, the close of each being dated, if our present
time-scale is reliable, at 15,000 to 20,000 years ago.
Both date and coincidence are decisive against the
theory. Although Croll's hypothesis achieved a
great popularity, geologists were not wanting who
considered the suggested causes to be utterly
inadequate to produce so radical a change of
climate. It happened that during the penultimate
period of extreme eccentricity, which began nearly
a million years ago, the astronomical conditions
were more favourable to glaciation than they have
been since. The question therefore arose why no
traces had been preserved in the deposits of that
time. Indeed, since the Cambrian, many hundreds
of glacial periods should have come and gone.
Had this been the case, and corroborative evidence
sufficiently convincing, the earth's chronology
would have been written boldly in its rocks. But
Croll's brave attempt to number the ages was
unsuccessful, and his attractive theory no longer
holds the field.

A gigantic experiment illustrating the very
conditions which Croll postulated has been

«— c eu

O *) <

c 2 0

<u rt „

Ill

« 2
E E

-.2

II £

•§.s §

JS.S 5?

E E

ASTRONOMICAL CONSIDERATIONS

recently shown to us by Prof. Lowell. For his
working model we must look to the sky and care-
fully watch the changing seasons of Mars. The
eccentricity of the orbit of Mars is much higher
than that of the earth could ever have been, and
moreover, the southern winter falls near aphelion.
The conditions for a permanent ice-cap over the
south polar regions are therefore ideal. The
actual facts are surprising, and on the accom-
panying plate Prof. Lowell has depicted them
admirably. During the winter a large snow-
cap collects around the south pole, but in
summer it is entirely dispersed. Around the
north pole the winter snow-cap is less exten-
sive, but all through the summer it never quite
disappears. While the long southern winter un-
doubtedly makes possible a greater accumula-
tion of snow, the hot short summer more than
compensates by its superior powers of evaporation.
A permanent and widely extended ice-field
evidently could not originate. This demonstration
of Croll's hypothesis in actual practice does not
present an altogether complete analogy to ter-
restrial conditions. The surface temperature of
Mars and the thermal properties of its atmosphere
may be widely different from ours, and the
excessive rate at which the polar caps diminish
in the spring clearly indicates the comparative
thinness of the deposit. But in spite of these
differences, the remarkable conclusion stands un-
assailed — that the evaporative power of the short
hot summer of the south exceeds that of the long
but cooler summer of the north.

It was stated above that since the culmination

THE AGE OF THE EARTH

of the last glacial epoch more than 15,000 years
have elapsed. Of the several methods which have
been employed to determine this period only one

FIG. 9.
Directions of Glacial Movement in Southern Scandinavia.

can be dealt with here. The present genial climate
has not greatly varied during the last 7000 years.
As we trace back the record of temperature still
farther a gradual fall can be discerned, accom-
panied by a wide extension of the ice. Fig. 9

ASTRONOMICAL CONSIDERATIONS

depicts the directions of movement of the last
continental glacier of Scandinavia and its
southern boundary across Jutland and the Baltic
provinces. As it retreated, it left the terminal
moraine known as the Baltic ridge, and its southern
limit was gradually pushed back till it extended
to what is now the coast of Scania. From this
point its recession has been followed in great
detail by De Geer, who has made a careful study
of the deposits which, extending from the Baltic
up to the Scandinavian ice-shed, mark the progress
of its annual retreat. Upon his observations he
has founded a system of geological chronology
which is of the greatest importance in that it marks
the first effort towards absolute accuracy.

Each spring and summer, as the glacier thawed, a
great deal of sand and clay was set free and
carried away in suspension by the numerous
streams which flowed from under the melting ice.
The coarser material, on reaching the sea, settled
down almost at once, but the finest particles of
clay, able to remain in suspension much longer,
were not completely deposited. Then came the
autumn and winter, and the freezing of the streams.
The sea received no further supply of sediment,
and the load of fine mud slowly settled on the sea
bottom to form a thin layer of pure clay, sharply
differentiated from the coarser bed below. The
following year the glacier retreated a few hundred
feet to the north, and the material then liberated
was sorted out as before and again deposited in two
well-marked seasonal layers. As this process con-
tinued year after year the area of deposit moved
northwards with the ice, and the annual layers of

THE AGE OF THE EARTH

sediment thus became superimposed one upon the
other like wedge-shaped tiles on a roof. The
width of each bed is generally less than thirty
miles, for even the finest mud cannot be traced
beyond that distance. This being the case, no
vertical section contains all the layers, and it is
rarely that more than a hundred can be counted in
one place. The total thickness of the recessional
deposit seldom exceeds thirty feet.

De Geer successfully attempted the difficult task
of counting the annual bands of glacial clay deposited
throughout the period of retreat from the Scanian
coast. Fortunately, any given set of layers can
be traced from one locality to another, and as each
ribbon of sediment dies out the higher beds are
followed up in the same way until the whole series
has been examined from bottom to top. The late-
glacial beds number about 5000, and the time
which has elapsed since the ice border reached the
eastern coast of Scania is therefore 12,000 years.*
The time of recession from the Baltic ridge to the
Scanian coast remains to be estimated. Much
of the record is hidden beneath the waters of
the Baltic, and in North Germany De Geer's
method has not yet been applied. It appears,
however, that the withdrawal of the ice was
not uniform. It began slowly and reluctantly,
but towards the north became more rapid. In the
region of Stockholm the retreat was five times as
fast as in Scania. We may therefore assume
that in the still earlier stages the time taken was
considerably more than that required for a retreat

* The post-glacial layers number about 7000.
42

ASTRONOMICAL CONSIDERATIONS

over an equal distance in Scania. The latter would
have occupied rather more than 2500 years, and
on this basis Sollas provisionally accepts 5000
years as the period during which the ice front was
driven back to the south coast of Sweden. In this
way the whole interval which has passed since the
culmination of the last glacial episode is determined
to be greater than 15,000 years, with 17,000 years
as a probable value.

Amongst the Archean rocks of N.W. Finland,
Prof. Sederholm has found in the Bottnian schists
and phyllites primary characters which are
strikingly similar to those of the banded glacial
clays. Under the microscope their textures are
distinctly clastic, and each composite stratum is
sharply divided into two thin bands, the coarser
one originally of sand and marking the beginning of
a new year of deposition, the finer one originally of
clay. Sederholm interprets the phenomenon on a
uniformitarian basis as indicating that, even in
those remote times, there was a marked difference
in the seasons. By measuring the thickness of
many thousands of annual layers, he finds their
mean thickness to be about five inches. The total
thickness of the banded phyllites amounts to
10,000 feet and the time they appear to represent
is therefore only 24,000 years. This conclusion is
probably far from the truth, for, as Sederholm
particularly emphasises, it is difficult to know what
is meant by the thickness of a deposit, so many
arbitrary and misleading elements enter into its
determination.

From De Geer's work, it might have been said at
first that a thickness of thirty feet was deposited in

THE AGE OF THE EARTH

a hundred years, but as the deposit was traced
over the country, it became equally evident, that
with no apparent thickening of the formation,
deposition had gone on for 5000 years, and con-
sidering the growth northwards from the Baltic
ridge, perhaps for 10,000 years. The whole
difficulty lies in determining which parts of a
formation are strictly contemporaneous. A
measurement of thickness is significant only in
relation to the immediate area of deposit and to
the rate at which it moves landwards or seawards.
The thickness of a formation and the maximum
thickness of the layers formed in successive years
may differ enormously, as De Geer's researches
have made so evident. For the same reason the
term rate of deposit is loose and misleading unless
it is clear to what it refers.

Another astronomical method of estimating
time, though embracing much longer periods, has
been applied by Gilbert to certain formations
in Colorado. The basin of the Arkansas River is
largely occupied by Cretaceous sediments, a succes-
sion of which is given in the adjoining diagram.
At the four stages marked A, B, C and D the
argillaceous shales give place to a calcareous type,
and in these there is a regular alternation of thin
layers of limestone and of calcareous shale. At A
the average thickness of a pair of beds is 1-5 feet,
and the number of repetitions is 15. At B the
limestones are more massive, but the parting
shales are very thin. Here again the average
thickness of two adjacent beds is 1-5 feet. The
limestones at C are less pure, and each, with
its associated layer of shale, amounts on the

THE AGE OF THE EARTH

average to about 2-7 feet. At D the succession is
similar.

To explain the remarkably regular alternation
of conditions which determined this uniform
rhythm of sedimentation, purely terrestrial causes
appear to be insufficient. Upheaval and subsidence
of the earth's crust, and the changing distribution
of land and sea, are characterised rather by their
irregularity than by any rhythmic sequence.
While there is undoubtedly a rough periodicity in
earth movements, yet it is discernible only on the
broadest scale and is out of all proportion to the
requirements of this case. Gilbert therefore
suggests an astronomical cause. Of the several
cyclic changes to which it is reasonable to appeal,
the annual revolution of the earth, and the varia-
tion of the eccentricity of its orbit, demand periods
which are in the first case too short (one year),
and in the second too long (91,000 years). As we
have already seen, the relation of the seasons to
the position of perihelion repeats itself about
every 21,000 years, and this astronomical cycle
seems better adapted to meet the case. The
climatic changes which accompany the precession
of the equinoxes might influence the character of
sedimentation in many ways. With a changing
circulation of winds and currents, argillaceous
material might be transported and deposited at
one time and calcareous at another. On land,
vegetation might predominate during part of the
cycle, and the surface waters would then dissolve
more calcium carbonate than during a period when
vegetation became sparse. At the same time
mechanical erosion would be impeded in the first
46

ASTRONOMICAL CONSIDERATIONS

instance, but would be more active in the
second.

It is evident that the shale was deposited more
rapidly than the limestone, for when the principal
deposit was calcareous the thickness is 1-5 feet,
whereas, in the case of the less calcareous beds,
it rises to 2-7 feet. In the normal shales a
conservative estimate of the equivalent thickness
would be 4 feet. Adopting the astronomical time
unit of 21,000 years, the rate of deposit would then
be of the order :

Limestone, i foot in 14,000 years.
Shale, i foot in 5,000 years.

On this basis the 3900 feet of shale in the Ben ton,
Niobrara and Pierre formations represent about 20
million years, for in this example the term thickness
seems to have a definite meaning. If the assump-
tions are correct, the duration of the whole
Cretaceous period must therefore be considerably
greater than this.

CHAPTER IV
THE WORK OF DENUDATION

Transference of material from land to sea — The denuda-
tion ratio — Weathering of rocks — The work of chemical
denudation — Summary of the data — Composition of the
saline matter in the oceans and of that annually carried to the
oceans — The work of mechanical denudation — Suspended
and bottom loads of rivers — Mississippi not a good average
case — Dole and Stabler's work in the United States —
Application to the whole land area — Total material re-
moved and rate of degradation of land — Marine erosion
— Types and quantities of sediments annually produced.

THE purely geological methods which have
been devised to investigate our problem
are of two kinds. The first attempts to apply a
time-scale to the sedimentary rocks and was,
historically, the earliest to be proposed ; the
second, due to Joly, deals with the accumulation
of salt in the oceans. The one is concerned with
material carried away from the land mechanically ;
the other with the material removed in solution.
The various agents of weathering, of which rain and
frost are the chief, disintegrate the surface rocks
and supply the rivers with their load of detritus.
The turbid condition of rivers when in flood,
heavily charged with alluvial matter, is a familiar
and convincing proof that the effect of erosion in
conjunction with the transporting power of running
water must always be to wear down the land areas.
In the dynamical study of denudation and
48

THE WORK OF DENUDATION

sedimentation the first essential is to know the
rate at which the rivers are working. Measure-
ments of their load of silt and dissolved salts and
of their annual discharge to the sea make it possible
to arrive at reliable estimates of their activity.

Incidentally, it is useful to determine the ratio
which solvent denudation bears to the whole.
The denudation ratio, as it may conveniently be
called, is the ratio of the load of dissolved material
to the total load carried both in solution and in
suspension.

The amount of material removed in solution
from the surface rocks is not quite the same as that
which is carried to the oceans. A small proportion
is abstracted from the over-ground circulation by
the waters which sink below the surface, and
while some of this is undoubtedly brought up
again through the agency of springs, it seems
possible, as Prof. Schwarz has boldly suggested,
that certain constituents, such as iron and mag-
nesium, may be permanently removed from the
earth's crust by downward migration. In the
denudation ratio this possibility is left out of
account as having no bearing on the study of
sedimentation.

A rough estimate of the denudation ratio may be
made by considering the weathering and decay of
rocks in situ. Soluble constituents are withdrawn
by leaching and a residue of the more stable minerals
and alteration products is left behind. Weathering
involves not only the abstraction of material but
also the introduction of fresh material from
external sources. Oxidation, hydration, and
carbonatisation are the most typical reactions, and

E 49

THE AGE OF THE EARTH

they must be allowed for in determining the pro-
portion of the original rock lost by solution. This
can be done approximately by assuming that
some element — aluminium being usually chosen —
has remained invariable during the course of de-
composition. From the analysis of a large number
of fresh rocks and of their altered equivalents, it
is found that on an average 30 % is dissolved,
leaving a residue of 70 %. The denudation ratio
ought therefore to be about 0-3.

The direct determination of the work of chemical
denudation requires three distinct sets of measure-
ments : (a) the annual discharge of rivers into the
oceans ; (b) the analysis of their waters ; (c) the
measurement of their drainage areas. Mellard
Reade was the first to point out the importance of
these factors in the study of dynamical geology.
In 1879 he collected such information as was
then available and deduced from it the quantity
of rock material annually removed from the whole
land area, his estimate being 5280 million tons. Sir
John Murray's corresponding estimate of 1887,
which was based on analyses of the waters of
nineteen of the world's principal rivers, was 4975
million tons.

Until 1909 this figure could not be improved
upon, but in that year there was published by the
United States Geological Survey the results of the
detailed and systematic work carried out by
R. B. Dole and H. Stabler. For the first time an
attempt had been made to measure the discharge,
drainage areas, salinity, and suspended load of all
the important rivers of a large continental area.
The estimates represent the averages of observa-

THE WORK OF DENUDATION

tions made daily for a year or longer, and in the
case of the discharge, the measurements extended
over at least seven years.

Seasonal variations and the effects of floods are
apt to be misleading, and a long continued series
of observations is necessary if their relative
importance is not to be over — or under — estimated.
The amount of material in solution varies but
slightly from year to year, and the average of one
year's results is within 10 % of the true mean value.
The annual discharge varies much more than this,
and still more inconstant is the load of suspended
material, which, in certain years, may differ from
the average value by 50 %. These figures indicate
how difficult it is to introduce exact measurements
into geology with any hope of finality.

A summary of all the best data now available has
recently been given by Dr. F. W. Clarke, and covers
about 28 million square miles of the drainage areas
of the earth. The details are given in the following
table :

CONTINENT.

DRAINAGE AREA IN
SQ. MILES.

SOLVENT DENUDATION.
TONS ANNUALLY REMOVED.

IER SQ.
MILE.

FROM WHOLE
AREA.

N. America. .
S. America . .
Europe ....
Asia

6,000,000
4,000,000
3,000,000
7,000,000
8,000,000

7°-5
45'5
90-0

75-o
40-0

423,OOO,OOO
182,000,000
27O,OOO,OOO
525,OOO,OOO
32O,OOO,OOO

Africa ..

28,000,000

6i'O

I,7O8,OOO,OOO

THE AGE OF THE EARTH

The total land area of the globe is estimated by
Murray at 55-7 million square miles, but of this
1 1 -5 million square miles are areas of internal
drainage, such as the Great Basin of the United
States and the Asiatic depressions, which con-
tribute nothing to the ocean. The circumpolar
regions, representing 4-5 million square miles, must
also be left out of account, and the remaining
397, or say, 40 million square miles, is that from
which the oceans are fed. If the figures given above
be accepted as typical, then the annual addition
of material to the oceans by solution amounts to
2440 million tons. Some of this, however, is
derived from the atmosphere — chiefly as carbon-
dioxide. Applying the necessary correction of
nearly 10%, there remains 2220 million tons as the
amount actually derived from the rocks. Murray's
estimate, it will be noticed, is almost exactly twice
that of Clarke.

The composition of the saline matter carried
to the oceans may be found by suitably weighing
each analysis of river water according to the
discharge of the latter. The general mean of all
such results is given in the table opposite, together
with the total amount of each substance in the
ocean. The small traces of elements other than
those listed are quite insignificant.

It is obvious from a comparison of these two
columns that the annual increment of dissolved
matter is not permanently retained by the oceans.
The greater proportion is precipitated by chemical
and organic agencies, and either becomes incor-
porated with detrital material or goes to form
individual sediments. The chief substances pro-

THE WORK OF DENUDATION

duced are calcium and magnesium carbonate,
gypsum, limonite, and silica. Rather more than
one-third of the carbonates appear to associate
themselves intimately with sands and muds. The
greater part of the remaining two-thirds is

ANNUAL ADDITION

TOTAL SALINE MATTER

CONSTITUENT.

OF SALINE MATTER

OF THE OCEANS IN

IN MILLIONS OF TONS.

MILLIONS OF TONS.

Si02

284

Fe2O3 A12O3

I,535,OOO,OOO

497

49O,OOO,OOO

156

I2,6l6,OOO,OOO

454,OOO,OOO

138

22,800,000,000

78,000,000

C03

857

80,300,000

NO 3

299

3,172,000,000

Total

2440

4I,23O,OOO,OOO

Annual discharge of river water

into ocean . . = 24-3 X io12 tons

Volume of ocean water . =307,496,000 cubic miles
Density of ocean water . = 1-026 (mean)

Mass of ocean water . . = i, 178, 270 x io12 tons

deposited as limestone on the continental shelves
(after being used by various organisms in shell-
making) in waters which are comparatively free
from terrigenous sediment. The abstraction of
gypsum from the ocean takes place at irregular

THE AGE OF THE EARTH

intervals under suitable conditions of concentra-
tion. Of the limonite and silica, the chief precipita-
tion takes place on the continental shelves where
they associate themselves with the detrital
sediments. The history of potassium is rather
obscure, but on the contrary, that of sodium
appears to be the simplest of all. That it is stored
up in the oceans is an assumption which is granted
as justifiable by most geologists.

The only new factor required in order to estimate
the mechanical work of denudation is the load of
material carried by the rivers. Besides the silt
transported in suspension, larger fragments are
carried by rolling along the stream bottom.
Measurements of the bottom load are lacking
except in a solitary case — that of the Missis-
sippi— in which it amounted to about 10 % of
the whole. It is difficult to define any precise
difference between bottom load and suspended
load, the former being only a limiting case of the
latter. When the water is fully charged with rock,
debris the highest proportions are found near the
bottom and sides, and at a point in mid-stream
at about one-third the depth from the surface
—the position of the stream lines of maximum
velocity. In clear water the rolling power reaches
its maximum value, for apart from fluid friction,
energy is expended in no other way. Most rivers
fall between these extremes. In making actual
measurements, samples are taken from repre-
sentative points in the river and used to give
the average over the whole section. In the
final estimate it seems probable that a large
proportion of the bottom load is accounted for.

THE WORK OF DENUDATION

Even if a correction ought to be applied it would
be pedantic to do so except for rivers which have
been under observation for several consecutive
years, since the variation from the mean annual
load is very great from year to year. The Nile
varies by 40 % and the Potomac by as much as
100 %. In the United States the mean variation
is about 50 %. With uncertain data of this kind
a correction of less that 10 % may safely be
disregarded.

Of all the rivers of the world, the Mississippi has
been most favoured by measurements of the kinds
required, and many estimates of the rate of
continental degradation, of the rate of deposition of
sediments and of the age of the earth have been
based upon them. There is no doubt, however,
that the Mississippi is working more rapidly than
any other river of importance in North America,
except, perhaps, the Colorado River. The high
declivity in the west, the Tertiary elevation of
the plains to which the streams are not yet
adjusted, and the abundance of easily eroded
glacial drift are all factors which promote this
activity. In the case of rivers other than those*
of North America for which data are available,
the same high rate of denudation obtains;
the Rhone and the Po, for example, being
amongst the most energetic workers in the world.
Generalising for the whole earth from these rivers
alone, would obviously give misleading results.
The work of Dole and Stabler again comes to our
aid, and in the following table their aggre-
gate measurements for the whole area of
the United States are tabulated. Similar evi-

THE AGE OF THE EARTH

dence for four widely separated rivers is also
given :

DRAINAGE BASINS.

AREA IN

SQ. MILES.

MILLIONS OF TONS OF
MATERIAL REMOVED
PER YEAR.

DENU-
DATION

RATIO.

IN SOLUTION

SUSPENSION

United States
Mississippi . .
Nile

3,088,500
1,265,000
1,100,000
150,000

34,800

24I-5
122
21
7'5

8-5

468

304
52
15
36

0'34
0-29
0-29

0'33
0-19

Uruguay ....
Rhone

Leaving out the Mississippi because of its
inclusion in the United States and weighing each
result according to the area over which it holds,
the mean denudation ratio is 0*31, a figure which
agrees very well with our previous estimate. ' If
now we use the denudation ratio to calculate the
material removed by mechanical denudation over
the whole land surface, we should not be far
from the truth. It is clear that if from the 40
million square miles which drain into the oceans
the quantity of material carried in solution
represents 0-3 of the total material removed, then,
as the former amounts to 2440 millions of tons
annually, the quantity carried away mechanically
must be 5700 million tons. That this figure is of the
right order is favoured by another consideration.
The mean elevation of North America is very
nearly that of all the land areas of the earth.
Moreover, according to Clarke's figures the rate of
denudation over North America is slightly higher

THE WORK OF DENUDATION

than the average for all the lands, but more closely
approaches it than does that of any other con-
tinental area. We may therefore take the rate of
denudation of North America as a fair average and
apply it with some confidence to all the drainage
areas of the globe. Doing this, the total amount
of suspended material annually discharged into the
oceans is computed to be 6000 million tons.

We may sum up the work of denudation in
round figures as follows :

Material annually removed in

solution 2500 million tons

Material annually removed in

suspension 6000 ,, „

Total 8500 million tons

These figures may also be expressed in terms of
the rate at which the land areas are being worn
down. By solvent denudation a degradation of one
foot in 30,000 years is implied, and by mechanical
denudation, one foot in 12,000 years. Taking both
together the average rate of denudation is found
to be one foot in 8600 years. It should be clearly
understood that individual areas may be lowered
at rates very different from this. The maximum
rate is attained in the Irawadi basin, one foot of
which is removed in 400 years. The Po is also an
exceptional river, and lowers its basin by one foot
in 850 years. On the other hand, in the Hudson
Bay district of North America the drainage only
carries away one foot in 47,000 years.

No minimum figure can be given, for wherever
deposition of sediment takes place on the land

THE AGE OF THE EARTH

areas the temporary rate of denudation locally
becomes negative. In making these calculations,
the density of rock material is taken as 2-6 ; the
weight of a cubic foot as 165 Ibs ; and the weight of
a cubic mile as 10,800 million tons. Although the
surface covering of loam or earth weighs only about
100 Ibs per cubic foot, the denser and more closely
packed underlying rock need alone be considered,
for it is by its decay and expansion that the super-
ficial blanket above is produced.

Our final problem is to determine the nature and
quantity of the sediments which are ultimately
formed on the continental shelves. This can only
be done roughly, but the results will suffice- to
serve our purpose. First of all, two serious
difficulties must be met before the way is open to
take this step. So far, marine denudation has
been left out of account. It is not yet possible to
make a wholly satisfactory estimate of the relative
magnitude of the supply of detritus captured
directly by the sea. The unknown factor is the
average encroachment of the sea upon the coasts.
For the British Isles, Croll suggested an average
of three feet per century, and the figure assumed
by Sir A. Geikie about the same time was ten feet
per century. A much later estimate by Prof.
Watts places the average retreat of the English
coast at a hundred feet per century. Along parts
of our East coast marine erosion is still more rapid
than this, the conditions being exceptionally
favourable. On the other hand, Geikie considers
that all the force of the Atlantic beating upon
the N.W. coast of Scotland may not wear it
away at more than one foot per century. What

THE WORK OF DENUDATION

the average between these extremes may be can
only be guessed at. If for convenience we accept
Geikie's figure as affording a likely average for all
the coast lines of the earth (125,000 miles), and if
the average height of the cliffs be taken as 150 feet,
then the mass of material annually removed will
be about 700 million tons.

The other difficulty is concerned with the annual
amount of material which remains in the oceans
in solution, and also of that which is deposited on
the ocean floor outside the limits of the continental
shelves. For the former, a knowledge of the age
of the oceans is necessary. Considering all the
evidence, the amount retained at the present day
seems to be about 200 million tons, but this is
certainly too high as a figure representing the
average increase throughout the history of the
oceans.

For the deep-sea deposits little more than a
guess is possible, although we can now approxi-
mate to the right order of magnitude by considering
the circulation of radium. The radium in the
material removed from the lands is redistributed
between the sediments on the continental shelves,
the deep-sea deposits and the water of the oceans.
Applying our present knowledge of the distribu-
tion of radium (see p. 131) the annual mass of the
deep-sea deposits is found to be about 1/30 of the
whole, i.e. about 300 million tons.

The difficulties can scarcely be avoided by
balancing them against each other. There still
remain 200 million tons (700 — 300 — 200) to be
added to the 8500 million tons already found as
the total for sub-aerial denudation. This gives us
59

THE AGE OF THE EARTH

8700, or as an extra safeguard, say 9000 million,
tons as the mass of sediment annually deposited
on the continental shelves. It is unfortunate that
to arrive at this figure an element of doubt should
be introduced by associating the results of careful
experimental work with the vague conclusions
just arrived at. The bugbear of the whole investi-
gation is marine erosion ; but if it is remembered
that the figures given in that connection are
meant to be suggestive rather than final, no
erroneous impression need be carried away.

If the sediments ultimately formed are shales
(20% quartz), sandstones (75% quartz) and lime-
stones (75% calcium carbonate), their proportions
will be as follows :

Shales . . . 70 % or 6300 million tons.
Sandstones . 16 % ,, 1440 „ „
Limestones . 14 % „ 1260

Total . . 100 % = 9000 million tons.

In the two following chapters the application
of denudational statistics to the measurement of
geological time will be considered.

CHAPTER V
THE SALINITY AND AGE OF THE OCEANS

Halley's proposal in 1715 — Joly's application of modern
data in 1899 — Further studies by Sollas, Clarke, and
Becker — Data of the problem — Corrections for dissemi-
nated sodium, wind-borne sodium, human agencies, and
marine erosion — Uniformity not capable of proof — Im-
portance of cyclic circulation of sodium — Origin of
chlorine now fixed in salt — Hour-glass method applied —
Land areas of the past and their elevation compared with
those of to-day.

IN 1715 the famous astronomer Edmund Halley
published a paper which he entitled, A short
Account of the Cause of the Saltness of the Ocean,
and of the several Lakes that emit no rivers; with
a Proposal by help thereof to discover the Age
of the World. He showed that since the water
removed from lakes by evaporation is perfectly
fresh, " the saline particles brought in by the
rivers remain behind, while the fresh evaporate ;
and hence it is evident that the salt in the lakes
will be continually augmented and the water
grow salter and salter." Applying the same
principle to the oceans, he wrote, "It is not
improbable but that the ocean itself is become
salt from the same cause, and we are thereby
furnished with an argument for estimating the
duration of all things." Two hundred years ago
it seemed hopeless to attempt to determine the
annual increment of salt added to the oceans, but
61

THE AGE OF THE EARTH

Halley used his argument " to refute the ancient
notion some have of late entertained of the
eternity of the world." The paper was completely
forgotten, until Dr. G. F. Becker again drew
attention to it a year or two ago.

As we saw in the last chapter, the analysis of
river waters has now made possible a determination
of the annual amount of material carried in
solution by the rivers into the oceans. T. Mellard
Reade was the first to contemplate the application
of solvent denudation to the measurement of
geological time, and the data he gathered together
in support of his arguments came as a great
surprise to those who had concentrated attention
merely on the mechanical work of erosion. No
independent advance, however, was possible until
1899, when Joly pointed out that of the many
elements which enter into the composition of sea-
water, sodium alone tends to accumulate. All the
others are sooner or later rejected, associating
themselves with the detrital sediments, or forming
chemical or organic sediments by their ultimate
precipitation. Joly then proceeded to use sodium
as the age-index of the oceans. He assumed that
the annual increment Nar of sodium added to the
oceans by all the rivers of the world has remained
practically constant throughout geological time.
If Na0 represents the total amount of sodium now
accumulated, the ratio Na0/Nar gives the time
which has elapsed since the oceans first existed
and denudation began to wear down the lands.
Joly's first estimate was from 80 to 90 million
years, and shortly afterwards he increased this to
100 million years. Sollas attacked the problem
62

THE AGE OF THE OCEANS

afresh in 1909, and from a most careful survey of
all the data and a detailed enquiry into every phase
of the subject, he concluded that the most probable
estimate of the age of the oceans would appear to
lie between 80 and 150 million years.

In 1910 a further study was made by Clarke and
Becker. The latter departed from the uniform-
itarian basis on which all the other calculations
had been founded. He inferred that sodium
accumulation progressed more rapidly in the past
than at present. All the original sodium must have
been derived from igneous rocks, and Becker
considers that at the time when the oceans were
first possible the surface of the earth must have
consisted exclusively of such rocks. At the present
day three-quarters of the land areas are covered
by sedimentary rocks which can supply no further
important additions to the sodium content of the
ocean. Assuming that the production of sodium
has been always proportional to the area of
igneous exposures, and that the total land area of
the globe has averaged 80% of the present area,
he finds that the age indicated is about 70 million
years.

We may now proceed to examine the problem in
detail. The fundamental data on which the method
is based may be summarised as follows :

Mean density of the ocean (Murray) . . 1-026
Volume of the ocean (Murray) 323,800,000 cu.miles

(Joly) 339^48,000 „
,, ,, (Clarke) 302,000,000 ,,

(Karsten) 307,496,000 „

THE AGE OF THE EARTH

Clarke now accepts the latter as being the best
estimate, and we therefore calculate that the

Mass of the ocean . . = 1,178, 270 Xio12 tons.
Total salinity of the ocean (Dittmar) . . . 3-5%
Sodium of the ocean (Dittmar) . . . .1-08%
Accumulated sodium, Na0 = 12,600 Xio12 tons.
Annual increment, Nar . = 156,000,000 tons.

From these values the quotient Na0/Nar gives
the age of the oceans to a first approximation as
80-8 million years. This figure, however, cannot
be regarded as final. Two of the assumptions on
which it is based are that all the sodium liberated
from igneous rocks is contained in the ocean, and
that all the sodium carried annually to the ocean
has been liberated from such rocks for the first
time. The obvious corrections to be applied will
increase the numerator of the ratio and decrease
the denominator, thereby increasing the age
estimate. The degree to which the numerator
must be augmented is, as far as we know, relatively
insignificant. Saline deposits, such as those of the
Stassfurt district, are only of trivial importance.
The salt of the ocean, if spread regularly over the
whole land areas, would form a layer about 120 feet
thick, and all the beds of rock salt which have
accumulated by evaporation become negligible
beside this vast quantity. The amount of sodium
in ground waters and disseminated through the
sedimentary rocks is to be taken into consideration
more carefully. Marine sediments at the time of
their formation are saturated w^h sea water, and,
when raised up to form land, they must, therefore,
be strongly charged with salt. The actual pro-

THE AGE OF THE OCEANS

portion of sodium abstracted from the ocean in
this way cannot at one time be very great, probably
not more than I % of the whole. Throughout
geological time, however, a certain amount of
sodium has been in cyclic circulation between land
and sea. A rough idea of the influence of this
circulation on the age estimate may be gained
from a knowledge of the total volume of the
sedimentary rocks, and of their average pore space.
Neither of these factors is yet known with the
desirable accuracy, but as far as present needs
are concerned, the total bulk of the sediments,
excluding deep-sea deposits, may be placed at
70 million cubic miles, and their pore space at
10 %. The latter figure is probably too low,
for fresh sediments have a pore space of 40 % to
50 % of their volume, and in consolidated sand-
stones 20 % is common. Many of the sediments
have been denuded and re-deposited time after
time ; on the average, probably three times. On
the other hand, if the ocean has progressively
increased in salinity, its average sodium content
must have been about half that of the present.
Basing our calculations on these considerations,
the total volume of sediments which have ever
existed is about 210 million cubic miles ; the pore
space to be filled by sea water would then be 21
million cubic miles. The total amount of sodium
precipitated within the rocks by evaporation could
not therefore exceed 420 million million tons, and
at the present rate of denudation its removal would
require nearly thr'e million years. To what
extent this result \v6uld be increased by absorption
effects, whereby interstitial salt solutions are

F 65

THE AGE OF THE EARTH

concentrated in order that they may be in equi-
librium with normal sea water, cannot be estimated.
This corrective factor would probably not be large.

A still more important cyclic circulation is
brought about by wind-borne sea salt. Fine
particles of spray are swept from the foaming crests
of waves by the wind and carried often many miles
inland. Near the coast the salt blown over the
land is naturally most abundant. It falls in sea-
fret and rain, and is ultimately returned to the
sea in the drainage from the land. Fortunately,
the amount of sodium supplied in this way can be
estimated by analyses of rainwater and a measure-
ment of the rainfall. As before, the cyclic sodium
must be excluded from that entering into the
denominator as primary sodium. Joly allows an
additive time correction of 10 %, Clarke of 7 %,
and Becker of 6 %. The time estimate is therefore
increased by a further five or six million years.

Clarke has suggested that the present rate of
sodium accumulation has been accelerated by
human agencies. Nearly six million tons of
sodium are annually produced as common salt,
and, as sewage and chemical refuse, much of this
is again returned to the oceans. Subtracting this
from the denominator, the age is increased by other
three million years.

A number of corrections may be made to decrease
the first rough estimate of the ocean's age. Solvent
denudation due to marine erosion must not be
forgotten. Experiments by Joly have shown that
sea water is several times (24—14) more active as
a solvent than fresh water. He attributes to its
action over the tide-swept strand — covering a

THE AGE OF THE OCEANS

total area of about 60,000 square miles — a supply
of sodium amounting to 3 % of that derived from
the normal drainage areas. This is a very liberal
estimate, and in applying a negative correction of
three million years we are granting all that can
reasonably be claimed.

Finally, there are other possibilities of uncertain
magnitude which deserve mention. The ocean
may conceivably have contained sodium before
the fluviatile contributions commenced. By
volcanic eruptions, sodium-bearing materials may
be cast widespread over the sea. Juvenile
waters expelled from igneous magmas may pre-
sumably afford a supply of sodium. The import-
ance of saline waters derived from the heated
interior of the earth has been particularly insisted
upon by the great geologist Suess. As a source of
sodium, such waters are probably of little moment,
for the whole amount of sodium in the ocean is
already more than explained by the erosion of
igneous rocks. Solvent denudation involves a loss
from average igneous rock of 1-57 % due to sodium
abstraction alone. According to Clarke, the
average sodium content of igneous rocks is 2-52 %,
and the amount retained by the detrital material
which goes to make up the sediments is 0-95 % of
the original rock. Now, if the total mass of sodium
which has accumulated in geological time is 12-6 x
io15 tons and this represents 1-57 % of the
igneous rock in which it originally occurred, it is
evident that the mass of the igneous rocks which
have been denuded away amounts to 800 x io15
tons. The mass of one cubic mile of rock may be
taken as 10,800 million tons, and hence the volume

THE AGE OF THE EARTH

of igneous rock which has been removed is 74
million cubic miles. The sediments derived from
the latter ought therefore to occupy a volume of
the same order. A number of independent
estimates of the volume of sediments existing on
the land areas have been attempted, and while
no great accuracy can be hoped for, it is significant
that all agree in assigning a volume somewhat
greater than 74 million cubic miles. The sedi-
ments hidden under the oceans would add still
further to the total. If any reliance at all is to be
placed on these estimates, it is clear that the sodium
in the ocean ought to be more than it is ; at any
rate, there would appear to be little room for
sodium derived from other sources, such as the
interior of the earth.

We may sum up the results of this discussion
as follows :

Approximate age, Na0/Nar=8i million years.
Correction for

(a) Disseminated Sodium = 3 million years.

(b) Wind-borne Sodium =6 „ „

(d) Marine erosion . = — 3

Age of the oceans ... 90 million years.

Another assumption on which this computation
is founded must now be examined ; that is, the
practical uniformity of the annual addition of
sodium throughout the period involved. Little
weight can be attached to the argument that the
source of supply has been gradually impoverished
in alkalies, for the soils from limestones, which are
68

THE AGE OF THE OCEANS

among the poorest of the alkali-bearing rocks, may
contain more sodium than do the soils derived
from granites. The sedimentary rocks being less
able to resist erosion are more rapidly disintegrated
and removed than are the igneous rocks. The
composition of streams considered in relation to
the rocks which they drain is of interest here.
Hanamann has shown that in the Elbe and its
tributaries three times more sodium is carried
away from a square mile of Cretaceous sediments
than from an equal area of granite. In the same
connection, it is well known that the waters
flowing through stratified rocks carry more salt
than those from igneous rocks. The greater
exposure of the latter in former land areas, if such
were the case, would certainly not contribute a
greater sodium income to the ocean than if
sedimentary rocks prevailed. Joly considers that
an approximate uniformity has been preserved
whatever the distribution of rock types may have
been. It would not, however, be difficult to prove
by actual analyses that a greater area of igneous
rocks would result in a reduction of the sodium
increment, and that the present rate may be too
high because of the predominance of sedimen-
taries.

This unexpected conclusion raises the question
of the validity of the method at its most critical
point. Can we be sure that the cyclic circulation of
sodium has been taken sufficiently into considera-
tion ? The only source to which the anomalous
proportion of sodium from sedimentary rocks can
be traced, apart from the factors already dealt
with, is constituted by the ground waters. These

THE AGE OF THE EARTH

waters often contain minute quantities of salt of
which the origin is very obscure. Whence came
the chlorine ? Igneous rocks themselves only
contain about 2 % of the chlorine necessary to
convert the liberated sodium into salt. The
emission of chlorine from volcanoes and solfataras
probably affords the necessary supply, but most
of this never reaches the ground waters directly.
Becker, dealing with the same difficulty in the case
of river water, calculates that 40 % of the annual
sodium increment is combined with chlorine of
which the origin cannot be traced. He accordingly
rejects this amount as not being of normal or
primary origin. If it has all been derived from the
oceans, which he regards as an extreme possibility,
it must be subtracted from the apparent income,
since it is simply a circulation of capital.

A maximum figure for the age of the oceans
(still on the assumption of uniformity) ought
theoretically to be attainable by accepting the
whole of the sodium-chloride of the rivers as cyclic.
We have the following figures :

Chlorine = 138 million tons.

Sodium combined with

chlorine = 87 ,, „

Sodium uncombined with

chlorine = 69 ,, „

Total Sodium, Nar =156 million tons.

The unchloridised sodium implies an age of 180
million years, an estimate which would in general
be condemned as excessive.

The difficulties presented by sediments and
70

THE AGE OF THE OCEANS

ground waters are almost insuperable, and it may
be safer to adopt a means by which they can
be entirely disregarded. The sodium lost from
igneous rocks amounts by mass to 1-57 % of the
latter. Igneous and metamorphic rocks are at
present exposed over one-fifth of the drainage
areas, or 8,000,000 square miles. Many of the
metamorphic rocks are partially or wholly of
sedimentary origin, and ought not to enter into the
computation. We have already seen that the
average rate of denudation is one foot in 8600
years, and at this rate, one cubic mile of primary
rock, or 10,800 million tons, would be denuded
away in 4-54 years. In one year the supply of
sodium would therefore be 37 million tons, and the
time required for the accumulation of the existing
store of sodium in the oceans would be 340
million years.

We meet with a curious discrepancy at this
point. The actual amount of unchloridised sodium
is 69 million tons, while that which is drawn from
the igneous rocks cannot exceed 36 million tons.
In each case sub-aerial denudation alone is being
considered, so that the failure to take marine
denudation into account does not affect the
matter. The figures imply that a great deal of
unchloridised sodium is drawn from the sedi-
mentaries. Even if the extreme assumption is
made for the moment, that the igneous rocks lose
all their sodium, 2-52 %, the annual supply could
only reach 60 million tons. The discrepancy is
evidently due to the facts that sediments freshly
formed from primary rocks retain a considerable
percentage of sodium, and that it is only after the

THE AGE OF THE EARTH

material has been re-assorted, perhaps several
times, that the element is withdrawn to the
extent indicated by Clarke's average figures. At
the present day, even when the igneous rocks have
been deprived of 37 million tons of sodium, the
sedimentary rocks, to bring up the total to 69
million tons, must lose more than half of their
primary sodium. It is interesting to observe that
if all the sodium now carried each year to the
oceans — 156 million tons — were derived directly
from the rocks, igneous and sedimentary, the
latter would be obliged to lose nearly twice as
much sodium as they actually contain ! This
remarkable conclusion indicates either that our
statistics of the annual production of sediments
are hopelessly wrong, or else that the chloridised
sodium is almost wholly cyclic. The latter
inference, supported as it is by the impossibility of
otherwise accounting for the chlorine, thus receives
further support.

In the application of the above data to the age
question, it has been tacitly assumed that the
present exposures of primary rocks are neither
greater nor less in area than the average of such
exposures since the oceans began.

It is impossible to know what the average
may have been, for while the original lands must
have been entirely formed of igneous or pseudo-
igneous planetesimal material, it is equally certain
that their area was but a fraction of that of the
present land areas. The evolution of the earth's
surface features has tended to intensify the differ-
ence between the ocean depths and the mountain
heights. The ocean basins have gradually deepened
72

THE AGE OF THE OCEANS

and become more stable, and their relative
permanence is generally admitted. The lands are
now more extensive than ever they have been ;
initially it is probable that only low and scattered
islands emerged from the shallow primitive seas.
We are faced here with many vague problems.
The volume of the seas may have been less than
it is to-day ; the original rocks were probably not
granitic in type, and were certainly poor in sodium ;
carbon-dioxide seems to have been the chief
atmospheric companion of nitrogen, and solvent
denudation would be correspondingly more vigor-
ous. How far all these considerations affect the
point at issue cannot be decided. To discuss
them would lead us into a complex maze of
speculation. The original basis of our calculation
may afford as near an approach to the truth as
could be arrived at by any other reasonable
hypothesis of changing land areas, and of their
composition.

On this view it is possible to calculate a minimum
figure for the age of the oceans, but it must be
remembered that the results based on an assump-
tion, which is merely a uniformitarian guess, are
themselves equally uncertain. If the igneous
rocks had lost all their sodium, we have already
found that the annual supply would be 60 million
tons, and the sediments would be destitute of that
element. The age would then be at least 210
million years. The contradiction between this
minimum figure and the maximum deduced from
the total unchloridised sodium, viz. 180 million
years, is explicable on one or both of two further
possibilities, that the average exposure of igneous

THE AGE OF THE EARTH

rocks in the past has been greater than that of
the present day, or that the amount of primary
sodium now being derived from the sedimentary
rocks is considerably above the average for the
past. Our interpretation of the data is made
particularly difficult because of the widespread
covering of glacial detritus, rich in felspathic
constituents and easily eroded.

The contradictory results become still worse
when it is remembered that the rate of denudation —
one foot in 8600 years — is probably too high to be
safely applied to areas of igneous rocks. Analysis
of river waters draining such areas indicate a rate
which is only about a half of the above. But even
this correction is surpassed in importance by one
arising out of the possibility that the present
standard of measurement may be misleading in
not being typical of geological time. The mean
height of the drainage areas is subject to much
fluctuation. By denudation it is steadily reduced ;
by earth movements, and particularly during
the periods of intense diastrophism, it may
become unusually great. It seems not unlikely
that we are now near a period of extreme con-
tinental elevation, and that the geological pro-
cesses which are thereby quickened up, cannot be
accepted as affording a true standard. This
suggestion will be dealt with further in the next
chapter.

The high figures — 210-340 million years — given
above, must not be supposed to possess any serious
value. The whole discussion merely serves to
betray the uncertainty of the method and the
doubtful applicability of even the most accurate

THE AGE OF THE OCEANS

data. For the present we can only conclude that
our knowledge of the part played by sodium and
chlorine in the constant redistribution of the
materials of the earth's crust is still lamentably
imperfect, and that quantitative deductions drawn
from it must be regarded as being purely pro-
visional.

CHAPTER VI
SEDIMENTATION AND GEOLOGICAL TIME

The maximum thickness of the sedimentary rocks —
Rate of deposit — Uniformitarian basis of the method as
usually applied — Arguments against the validity of this
assumption — Present a period of land extension and con-
tinental elevation — Present geological rates not true
standards — Ideal sedimentation curve and rates of de-
posit— Difficulties in the application of the data — Sum-
mary of time estimates based on this method — The hour-
glass method applied to the accumulation of sediments
and of calcium carbonate.

THE most familiar method of estimating
geological time is based upon the total
observed thickness of stratified rocks and the
rate at which they may have been deposited.
Our knowledge of the so-called maximum thick-
ness of each of the stratigraphical systems has
been carefully summarised by Sollas, to whom we
owe the following table.

Maximum Thickness of the Geological Systems.

FEET

Recent and Pleistocene . 4,000

Pliocene 13,000

Miocene 14,000

Oligocene .... 12,000

Eocene 20,000

63,000 feet.

SEDIMENTATION

Maximum Thickness of the Geological Systems — contd.

FEET

Cretaceous .... 44,000

Jurassic 8,000

Triassic 17,000

69,000 feet.

Permian .... 12,000

Carboniferous . . . 29,000

Devonian .... 22,000

- 63,000 „

Silurian 15,000

Ordivician 17,000

Cambrian 26,000

58,000 „

Algonkian .... 82,000 82,000 „
Archean ? ?

Total 335,ooo feet.

The rate of deposit of sediment on the ocean
floor is a factor over which there has been much
dispute. The rate varies between very wide limits,
according to distance from the shore and from
the mouths of great and active rivers. What is
generally sought is to give an estimate which
will correspond to the maximum thickness of
accumulated material, i.e. an estimate of the
average maximum rate of deposit. The same rate
is then applied in turn to the whole of the
geological column in the hope that no serious
error will be introduced by the assumption of
uniformity. This attitude was taken up in
defence against the attacks of Kelvin and his

THE AGE OF THE EARTH

followers. The tendency to invoke more active
geological agencies in the past, greater floods
and tidal waves, a more stupendous upheaval
of mountains and more violent volcanic erup-
tions, did not commend itself to most geologists.
Geikie stated the case very clearly in his eloquent
address of 1892, when he affirmed that " the
geological record furnishes a mass of evidence
which no arguments drawn from other depart-
ments of Nature can explain away, and which, it
seems to me, cannot be satisfactorily interpreted
save with an allowance of time much beyond the
narrow limits which recent physical speculation
would concede." But while the conception of
greater activity in the past met with little favour,
the assertion of uniformity was as far as geologists
dared to go. No one suggested that we might be
living in an age of more than average activity.
Yet there are many reasons which favour this
hypothesis in preference to the alternative views.
That the average land area of the past was
less than that of to-day has already been
stated. According to the palaeo-geographical re-
searches of Mr. C. Schuchert, the mean area of
North America since Cambrian times has been
four-fifths of its present area. In the case of
the other continents a smaller fraction would
probably be more representative. Of very much
greater importance is the fact that the aver-
age height of the land areas above sea-level
has often been less than it is, so that the present
average is excessive when viewed from the
broader standpoint of geological time. The chief
defect in the time estimates based on the rate

SEDIMENTATION

of sedimentation lies, according to Chamber-
lin, in the too full dependence on standards derived
from the geological processes now in action. It is
tacitly assumed that current rates are representa-
tive, or that the departure from the mean rate is
not such as to involve any grave error. Joly, for
example, in discussing the divergent evidence of
geological processes and radioactive minerals,
points out that to bring the different methods into
agreement we must assume " that the rivers are
now bearing to the sea about 14 times the average
peicentage of the past — not less than 9 times."
Then he says, " It seems quite impossible to find
any explanation of such an increase."

In the present high relief of the earth's surface
at least a partial explanation may be found. Prof.
Chamberlin writes in a private communication to the
author : " Because of the relatively high gradients,
the wash of clastic material from the slopes and its
deposition in the basins, as well as the transfer of
salts to the sea, are to-day more rapid than in
average times. We seem to be at, or near, one of
the great extremes of intensification of the processes
of solution and degradation. And so, whether
conclusions are based upon degradation and
clastic deposition, or upon solvent action and the
accumulation of solutes in the sea, the present
rates are high rates, and if these are made the basis
of time estimates, the estimates are minimum
ones. There are abundant evidences that periods
of base-levelling have occupied a notable part of
geological time. There is cogent evidence that the
Archean and Proterozoic (Algonkian) terranes
were reduced well towards base-level in pre-
79

THE AGE OF THE EARTH

Cambrian times, and that subsequently extensive
base-levelling clearly seems to have intervened at
repeated intervals. To me the evidence seems to
support the existence of a dozen or a score of
stages of peneplanation, some of which appear to
have made a notable advance toward complete
base-levelling."

During those intervals, when the average
continental height was low, denudation and
deposition would proceed very slowly. How
slowly, we have no adequate means for determining.
A very careful study of drainage basins with
reference to their mean elevation would be a step
towards a sounder method than it has yet been
possible to apply. That the departure from present
rates, in past time, may have been very consider-
able, is indicated crudely by a very simple calcula-
tion. It has been found experimentally that the
carrying power of water varies as the sixth power
of its velocity. Roughly, we may say it varies as the
sixth power of the mean square of the heights of the
drainage basin from which it finds its way to the
sea. If this, were strictly true, then, if the present-
day contours of the land were reduced to half their
value, the power of removing material would be
reduced to less than one four- thousandth. Of
course, other factors would begin to operate which
would prevent the attainment of any variation as
extreme as this. Nevertheless, our faith in the
value of present standards as applied to geological
time cannot but be seriously shaken. Chamberlin
feels warranted in thinking " that the substitution
of mean velocities of denudation, deposition, and
saline accumulation, if it could be made to ap-

SEDIMENTATION

proach the realities of the case, would have the
effect of multiplying by a considerable figure the
best estimates that have been made on the basis
of current velocities." Other factors tending to
increase the age estimate are numerous, but
comparatively insignificant beside those already
briefly discussed.

In the last chapter we concluded with an estimate
of the quantity of different types of sediment
which are now annually deposited.

Shales .... 6300 million tons.
Sandstones . . . 1440 „ ,,
Limestones . 1260

9000 million tons.

If it be assumed that all the arenaceous sedi-
ments form littoral, deltaic and estuarine types,
i.e. that they are concentrated over an area of say
100,000 square miles, the rate of accumulation
would be one foot in 150 years, or dealing only with
fresh unconsolidated sediment, of one foot in about
100 years. But these figures are obviously too
high, for a great deal of sandstone comes under
shallow water marine types. Fortunately, in the
deltaic deposits of two important rivers human
remains of recognised age are found buried ; and
from measurements of the thickness in each case
it is known that the Nile has deposited loose
sediment at one foot in 320 years, and the Po at
one foot in 174 years.

We have here, as in all deltaic deposits, a mcde
of growth analogous to that of the glacial clays
studied by De Geer. It by no means follows that

G 81

THE AGE OF THE EARTH

these rivers will continue to raise their beds at the
same rates, for the main growth of deposit is not
upwards, but seawards.

To get a rough idea of the rate of sedimentation,
the ideal section opposite has been constructed.
The continental shelves are assumed on the
average to be representable as a band of 100 miles
in width fringing a coastline of 100,000 miles.
The average thickness of a deposit laid down
according to this plan would be about 0-4 of the
greatest thickness, the latter, on the scale of the
diagram, having reached 400 feet. In the par-
ticular case illustrated the land is slowly sinking,
and as the sea encroaches upon it the deposits
gradually overlap.

If deposited over one square mile, as consolidated
sediments of density 2*5, our 9000 million tons of
material would form a rectangular mass 4570 feet
in thickness. Being deposited over the ten million
square miles of the continental shelf, the average
thickness of the layer is 0*000457 feet, correspond-
ing to a rate of deposit of one foot in 2200 years.
The maximum rate is therefore one foot in 880
years. If this figure be applied to the total thick-
ness of the geological column, then, disregarding its
imperfection, the time implied would be about
300 million years.

The distribution of conglomerate, sandstone and
shale is sufficiently indicated in the diagram. It
is difficult to know how to distribute the argillace-
ous and calcareous types. They are mutually
exclusive, for limestone cannot form by organic
agencies in places where mechanical detritus is
being deposited. They are therefore taken
82

H- 2

THE AGE OF THE EARTH

together, on the basis that where one is not being
formed the other is.

The following table gives the rates of deposit
at various distances from the shore, for the case
illustrated in Fig. n :

YEARS FOR DEPOSIT OF ONE FOOT.

JJISTANCE FROM
SHORE IN MILES.

FRESH

DENSITY = 1*8.

CONSOLIDATED

DENSITY = 2 '5.

c
£

2,780

2,000

•1 •

1,670

1,200

3°

1,120

880

4°

1,4.10

1,030

5°

en
tr
P»

2,280
5,210

1, 660
3,750*

10,420

7,500*

13,900

IO,OOO*

20,850

15,000*

100

41,700

3O,OOO

* Cf. Rates of deposit of the Cretaceous sediments of Colorado,
p. 47-

These figures, however, have but little value, for
there is no single law of deposition. The effects of
ocean currents, of earth movement, and of the
presence or absence of great rivers should all be
considered, and they provide a problem so com-
plex that as yet it is hopelessly t^ond a general
solution. Difficulties are encountered at every
stage ; not only are the estimated rates of doubtful
value, but their application is discredited by our
ignorance as to what constitutes the real maximum
thickness of sediments.

SEDIMENTATION

As already indicated, the latter difficulty is due
to the tendency for successive beds to overlap
while the cycle of deposition is running its course.
As was admirably stated by Prof. Watts in his
Presidential address to the Geological Society in
1911, deltaic deposits gradually extending sea-
wards are more characteristic during periods when
the land is being elevated relative to sea level.
On the contrary, during periods of depression
estuarine conditions prevail and the beds grow
•landwards.

The same problem has been attacked by
Sederholm from a rather different point of view.
He says, " As the layers successively formed cover
each other like scales or roof-tiles, no vertical
section contains them all. If we mean by maxi-
mum thickness the sum of the maxima of the layers
formed in successive years, it certainly measures
millions of feet." In this case the rates of deposit
as ordinarily found would not, of course, be
applicable.

Before proceeding farther, it may be well to
review the various estimates of time which have
been founded upon the geological method. The
earlier geologists believed that the sediments were
deposited widespread over the ocean floor and the
rate of deposition was therefore taken as even less
than that of denudation. Then came the Chal-
lenger expedition in 1872-5, and it became certain
that the formation of all except the less im-
portant deep-sea deposits takes place almost
entirely on the submarine continental shelves.
A considerable modification of the estimated
rates of deposition was then made necessary

THE AGE OF THE EARTH

and the time periods were correspondingly
shortened.

DATE.

AUTHOR.

MAXIMUM
THICKNESS

IN FEET.

RATE OF

DEPOSIT
YEARS FOR

ONE FOOT.

TIME IN
MILLIONS
OF YEARS.

i860

Phillips

72,000

1332

1869

Huxley

IOO,OOO

1000

IOO

1871

Haughton

177,200

8616

1526

1878

Haughton

177,200

2OO

1883

Winchell

—

1889

Croll

12,00.0*

6ooof

1890

de Lapparent

150,000

600

1892

Wallace

177,200

158

1892

Geikie

100,000

730-6800

73-680

1893

McGee

264,000

6000

1584

1893
1893

Upham
Walcott

264,000

316

IOO

45-7°

1893

Reade

31,680*

300of

1895

Sollas

164,000

100

1897

Sederholm

—

35-40

1899

Geikie

—

IOO

1900

Sollas

265,OOO

IOO

26-5

1908

Joly

265,OOO

300

1909

Sollas

335.800

IOO

* Spread evenly over the land areas.
t Kate of denudation.

Most of these estimates are little more than
rough guesses. We do not know how much of the
story is lost to us, or how much is hidden away.
The time which has usually been regarded as
expressing the geological requirements most
adequately is 100 million years. The fanciful
figures arrived at by Winchell, and McGee (who
suggested a probable age of 6000 million years)

SEDIMENTATION

are merely illustrations of how the data could be
twisted to produce impossibly extreme results.
The latest estimate, due to Sollas, includes an
allowance of 25-4 million years for the duration of
pre- Cambrian time — the same period as that
which has apparently elapsed since. A further
allowance is made for unconformities, those gaps
in the sequence which are unrepresented by
sediment. Taking the great unconformities as
probably numbering six, each being equivalent
to 40,000 feet of sediment, 24 million years are
added. For minor unconformities and interrup-
tions in the record other 5 millions are granted
and the total is thus brought up to 80 million years.
In an attempt to free ourselves from the
difficulties with which this method is beset, we
may adopt a mode of procedure similar to that
followed in the last chapter. It was there assumed
that the total volume of the sediments which have
ever existed amounts to some 210 million cubic
miles. The present annual supply of sediment
when ultimately compressed and consolidated
would occupy 0-83 of a cubic mile. If the
present rate of accumulation were reliable,
geological time would then be of the order 250
million years. A still nearer approach to the
truth may be made by calculating on a uniformi-
tarian basis how long the existing sediments,
which we placed at 70 cubic miles, have taken to
form. From the 8,000,000 square miles of igneous
and metamorphic rocks, one cubic mile would be
denuded away in 4-54 years, which implies that
one cubic mile of consolidated sedimentaries
would be formed in about five years. The age

THE AGE OF THE EARTH

then works out at 350 million years. Making a
further correction for the slower rate of denuda-
tion of igneous rocks, this figure may perhaps be
doubled. Finally, there is the correction for
average rate in place of present rate, and to what
extent this would increase the estimate it is
impossible to say.

We may revert to the maximum thickness of
the sedimentary rocks to support the estimate of
the total volume of sediments which has ever
existed. On the basis of the sedimentation curve,
the sediments have been deposited on an area of
10 million square miles, and if laid down every-
where at their average maximum thickness, 60
miles, they would cover about 0-4 of that area.
The total volume which can ever have existed,
leaving unconformities out of the question, is
therefore, 60x10,000,000x0-4 cubic miles, or 240
million cubic miles.

Finally, a crude estimate may be based on the
amount of calcium carbonate which has accumu-
lated in geological time. Several estimates of the
volume of limestones in existence have been made,
e.g.:

Dana . . . 18-40 million cubic miles.

Reade . . 10-00 ,,

Van Hise . 6-25 ,, ,, ,,

The limestones now forming make up 14 % of
the total sediments which collect on the continental
shelves. On the land the proportion must be
lower than this, because limestone is denuded at
a rate well above the average. If we take lime-
stone formations at 10 %, the volume would be
88

SEDIMENTATION

about 7 million cubic miles, a figure not far from
that of Van Hise. The calcium carbonate may
now be estimated. Limestones contain on an
average 75 % and shales and sandstones together
about 7 %. The total volume, calculated at
density 2-6, would therefore be in round figures
10 million cubic miles.* Igneous rock contains
3*43 % of calcium, and if in the process of denuda-
tion all of this is dissolved and removed, the
present rate of production of calcium would be
equivalent to i cubic mile of calcium carbonate in
32 years, on the same basis as in previous calcula-
tions. The time estimate at this rate would be
320 million years.

We may now sum up our various results as
follows :

1. Accumulation of Sodium. MILLION YEARS.

(a) Uncorrected quotient Na0/Nar . 80-8

(b) Partially corrected Na0/Nar . . 90

(d) Primary sodium alone . . . 210-340

2. Accumulation of Sediments.

(a) Maximum thickness .... 300

(b) Total volume which has ever

existed 250

3. Accumulation of CaCo3.

(a) Total volume now existing . . 320

Not one of these estimates is to be regarded as

* Leith finds 12-5 - 22 million cubic miles.
89

THE AGE OF THE EARTH

final ; the uncertainties are too many and too great.
The whole trend of this chapter has been to show
that whatever may be the true reading of the
hour-glass of denudation and deposition, it ought
probably to be very much higher than has been
generally assumed.

CHAPTER VII
RADIOACTIVITY

Geological application — Rontgen rays — Becquerel rays
— Mme. Curie and the discovery of Radium — The gold-
leaf electroscope — The nature of a-, /3- and y- rays — Helium
and its production from radium — The heat output of
radium — Transformation of the radium atom — Disinte-
gration theory — The half-life period of radium and its
generation from uranium — The uranium, thorium and
actinium families — Direct measurement of the heat output
— Radioactivity independent of external influences — •
Pleochroic haloes — Measurement of radium in rocks and
minerals.

IN the limited space at our disposal, only a
brief outline of the salient facts of the new
science of radioactivity can be given. It is
necessary to give some account of these recent
discoveries because of their immediate application
to the study of the earth itself. In a science like
geology, the function of which is to study the
consequences of energy transformations in the
earth's crust during the long ages of her gyrations
through space, the recognition of sources of
energy previously undreamed of is a stimulant to
research which must profoundly affect the inter-
pretations of the phenomena we observe. For our
present purpose it will be necessary to review only
those facts which have a bearing on the question
of the earth's age, and of its thermal history.
In 1895 Rontgen gave his first account of

THE AGE OF THE EARTH

A New Kind of Radiation. This extraordinary
radiation, now familiar to us as the X, or Rontgen
rays, revealed itself by its remarkable property of
passing through objects which are entirely opaque
to ordinary light. It was found, for example, that
a photographic plate was not protected from its
influence by being wrapped in black paper, but was
affected just as though it had been exposed.

This discovery awakened the deepest interest
amongst all scientific workers, and the idea arose
that phosphorescence and the Rontgen rays
might in some way be vitally connected. Certain
substances, after being 'exposed to sunlight, are
able to shine in the dark with a phosphorescent light,
and it was this radiation which was examined from
the new point of view. Working on these lines, with
uranium salts, Becquerel found, almost by accident,
that they gave out rays which were capable of
penetrating black paper and making themselves
evident by their effect upon a photographic plate
wrapped within it. This observation in 1896 marks
the commencement of the harvest of wonderful
discoveries which have revolutionised our con-
ception of the structure of matter.

The emission of rays was found to be an inherent
property of uranium and entirely independent of
the phosphorescence exhibited by certain of its
compounds. Physical and chemical changes made
no difference ; the radioactive properties evidently
belonged to the atom itself, for they could be
detected under whatever conditions uranium
happened to be present. Mme. Curie at once began
an exhaustive examination of all the other known
elements, and came to the conclusion that thorium
92

FIG. 12.
Taken in the dark by its own rays.

FIG. 13.

Taken in ordinary daylight
PHOTOGRAPHS OF PITCHBLENDE.

RADIOACTIVITY

alone possessed radioactive properties similar to
those of uranium. In the course of her researches,
she found that uranium-bearing minerals such
as pitchblende (uraninite) were far more active in
their emission of rays than could be accounted
for by the uranium present. Systematically
following up so significant a fact, she traced the
activity to the bismuth and barium separated from
these minerals. In 1898 two new elements were
announced. One, associated with bismuth, was
called Polonium ; the other, found in minute
quantities with the barium, was appropriately
named Radium.

The spontaneous radiations of these elements,
collectively known as the Becquerel rays, were
soon distinguished from the Rontgen rays by
their greater complexity. Their analysis is
largely due to the scientific genius of Rutherford,
who has classed them into three well-marked types,
the Alpha or a-, Beta or /?-, and Gamma or y-
rays. The instrument chiefly employed in their
detection is the gold-leaf electroscope, which not
only is simple in construction, but is capable of
extreme sensitiveness. Under ordinary conditions
the air is a good insulator, and a charged electro-
scope loses its charge only very slowly. However,
if radium is brought near the instrument, the dis-
charge proceeds much more rapidly, the effect of
the Becquerel rays being to ionise the air and so
render it conductive to electricity. In the electro-
scope this effect can be readily observed and
accurately measured, for the leaf falls at a regular
rate, which depends on the intensity of the radia-
tion. The sensitiveness of this method so far

THE AGE OF THE EARTH

surpasses that of the most delicately adjusted
balance that by its means a quantity of radium
can be detected which would need to be multiplied
thousands of millions of times before the balance
would turn to its weight.

It is easy — by interposing between the electro-
scope and the radium a gradually increasing thickness
of suitable material such as thin sheet aluminium —
to establish the existence of three kinds of rays
which differ greatly in their power of penetrating
matter. The y-rays are, on the average, about
a hundred times more penetrating than the /St-
rays, and these in turn are equally the superior of
the a-rays. In each case the penetration in
different materials depends approximately on their
density. The three types can also be distinguished
by their behaviour in a strong magnetic field.
Giesel showed that the /3-rays were deflected in
such a way as to indicate that they consisted of
particles carrying a negative charge of electricity.
The a-rays were much more difficult to deflect,
but Rutherford successfully turned them from
their path by applying a very intense field, and
from the direction of their deflection he concluded
that they consisted of positively charged particles.
The y-rays have, however, resisted all attempts
made to alter their course. Their real nature is
not yet beyond dispute, and from the great
similarity between their properties and those of
the Rontgen rays they are generally considered
to be, like the latter, either electro-magnetic pulses
in the ether, or uncharged corpuscles of a kind
not yet understood.

When an electric discharge is passed through

RADIOACTIVITY

a high- vacuum tube, rays are emitted from the
cathode which, on reaching the walls of the tube,
produce a yellowish-green phosphorescence. The
cathode rays are identical with /?-rays in every
essential respect, and both are proved to be tiny,
negatively charged particles, called electrons.
The velocities of both cathode and ^-particles are
inconceivably high ; in the latter case even
approaching that of light. Now Rontgen rays,
whatever may be their true nature, are set up by
the sudden stoppage of electrons when they
impinge upon the anti-cathode of a vacuum tube.
In the same way it is supposed that the /2-rays,
by their sudden expulsion, give rise at the same
moment to y-rays. It is a significant fact in this
connection that the /?- and y-rays are always
associated together, whereas the a-ray may be
quite independent.

The story of the brilliant researches which
culminated in the measurement of the mass of an
electron, and of the charge it carries, is one of the
most fascinating in the annals of science. But
here we must be content with the bare state-
ment that the electron has only 1/1700 of the
mass of a hydrogen atom. The application of
similar methods to the a-particle has demon-
strated that it is of atomic dimensions, and consists
either of a hydrogen atom with a single ionic
charge, or of a helium atom with twice that
charge. Other evidence decides conclusively in
favour of the second alternative.

Of all the elements, helium has probably the
most interesting history. It was first discovered
in the sun during the eclipse of 1868, but it was not

THE AGE OF THE EARTH

until 1895 that it was detected in terrestrial
materials. In that year Sir W. Ramsay identified
it in uranium- and thorium-bearing minerals by
means of its highly characteristic spectrum. Even
then, before radioactivity had been recognised, the
association of helium with these elements had
forced itself upon his notice. In 1902 the genetic
relationship between helium and certain of the
radioactive elements was predicted by Rutherford
and Soddy. During the following year Ramsay
and Soddy working in conjunction sought for, and
actually observed, its generation from radium.
Using a radium preparation from which helium
was at first entirely absent they were able to
prove, with the aid of the spectroscope, that in
the course of a few months suificient of the gas had
been generated to be identified. This important
discovery has received abundant confirmation
during the last decade, and to-day we even know
the number of helium atoms which a gram of
radium emits in a given time. Moreover, the
production of helium has been demonstrated from
uranium, thorium and actinium, and from most
of the other radioactive elements which are
characterised by the emission of a-rays. The
rate of generation of helium will be considered in
more detail in Chapter X, where it finds an im-
portant application.

An observation of fundamental importance was
made by Curie and Laborde in 1903 when they
discovered that radium is able to maintain a tem-
perature above that of its environment. The source
of this emission of heat lies in the kinetic energy
of the Becquerel rays. Projected from radium,
96

RADIOACTIVITY

they are sooner or later completely absorbed by
the matter through which they penetrate. Simul-
taneously, their energy is given up to the molecules
with which they collide, and it therefore reappears
as heat. By far the greater proportion — over
85 % — of the energy is carried by the relatively
heavy a-particles which move with velocities
nearly one-tenth that of light. Knowing their
masses and velocities, and the number liberated
per hour from a gram of radium, it is a simple
dynamical problem to calculate their total kinetic
energy. Expressing this quantity in heat units,
it is found to be equivalent to an hourly production
of 113 calories. (Ap. A, p. 177.) As we shall see
later, this figure is in close agreement with the
results of direct measurements.

The most remarkable feature about these
phenomena is that they appear to continue
unceasingly. Year after year the spontaneous
production of helium goes on, accompanied by a
steady evolution of heat. But what of the atoms
that remain ? They evidently cannot continue
to be the same element after having given up part
of their energy and expelled from themselves the
material particles of the a- and /3-rays. A close
examination reveals the genesis of a new element
— a gaseous substance known as radium emana-
tion. Its atom represents the residual fraction of
the radium atom after the separation of a single
a-particle. The emanation is highly unstable ; so
rapidly does it give up energy and liberate helium
that its original intense activity decreases by half
every four days. In its place arises another pro-
duct, radium A, of still more transitory existence.

H 97

THE AGE OF THE EARTH

A long succession of similar transmutations has been
traced, each accompanied by a setting free of energy.

In 1902 Rutherford and Soddy advanced the
theory of the disintegration of the radioactive
atoms. The radium atom is essentially unstable
and ultimately breaks up, explosively emitting
a- and /3-particles. Each atom has a certain
expectation of life which distinguishes it from the
atoms of other radioactive substances ; but such is
the distribution of instability that a constant
proportion of the total number of atoms present
is always breaking up. Out of a million atoms of
radium a definite number disintegrates each
second, and the proportion has been proved to be
invariable in every case it has been possible to
test. From element to element it is this propor-
tion, and therefore the average promise of life,
which varies. An equilibrium between formation
and transformation is gradually attained, and once
it is established a given quantity of radium pro-
duces as many atoms of emanation as the emanation
produces of radium A, and so on. When in equi-
librium the longer-lived radio-elements are present
in greater quantity than those of shorter lives
in order to compensate for their comparatively
slow rate of decay, and so to enable them to keep
pace with their more rapidly changing associates.

We may now calculate the average life period of
radium. The volume of emanation in equilibrium
with one gram of radium has been measured and
found to be 0-58 cubic millimetres — less than the
volume of a pin's head. The rate of change into
radium A also lends itself to direct measurement,
and from these two quantities it can be shown

RADIOACTIVITY

(see Ap. A, p. 179) that in one year 1/2600 of the
radium originally in existence must undergo trans-
formation. The same result can be arrived at
more directly. We know the number of atoms in a
gram of radium and also the number of helium
atoms expelled in the course of a year. The pro-
portion which has suffered disruption follows at
once, and is found to be 1/2500.

From evidence of this kind it is concluded that
after about 1850 years only half of any given
quantity of radium then maintains its identity as
that element. Now it is clear that radium must
have a progenitor by which it is formed as fast as
it decays, or otherwise there could now, after the
lapse of millions of years, be none in existence.

That progenitor is undoubtedly uranium — an
element which disintegrates so slowly that its
half-life period is three million times as long as
that of radium. This association is revealed by the
study of radioactive minerals. For every gram of
uranium there exists in equilibrium with it,
3*4 X io~7 grams of radium. The constancy of this
proportion points to a genetic connection and
admits of no other interpretation. The generation
of radium has been experimentally verified by
Soddy, who observed it after a number of years
in a uranium preparation originally quite free from
it. In practice this work is complicated, and the
desired effects are retarded, by the intermediate
production of another radio-element, ionium,
which has an average life many times longer than
that of radium. The time taken to establish
equilibrium is therefore very long. The primary
minerals from which the equilibrium ratio of radium

THE AGE OF THE EARTH

to uranium is deduced, have existed for hundreds
of millions of years and so far as they have
remained unaltered by percolating waters they
afford a safe guide. But in secondary minerals, like
autunite, the age of which is often to be reckoned
only in thousands of years, the equilibrium stage
has not always been reached, and the ratio between
the two elements is consequently somewhat lower
than that found in the older minerals.

In the accompanying diagram the complete
family of uranium is summarised together with
the half-life period of each member and the rays it
emits. Another series of elements related in the
same way is derived from thorium which is a
parent of even longer life than uranium. As to
the final products of transformation, which, being
stable, ought to survive and in the course of
time to accumulate, helium is the only one
which has been directly observed. That lead is
the end product of the uranium series is most
probable, as we shall see in a later chapter. Of
the stable element to which the thorium series
ultimately leads we are still in ignorance. Neither
lead nor bismuth, which suggest themselves by
their appropriate atomic weights, satisfy the
conditions, for their presence in a mineral bears
no relation to the quantity of thorium (see p. 190).

The actinium group of radio - elements calls
for little notice here. Boltwood finds a fixed
ratio between actinium and uranium in minerals,
and the former would therefore appear to be a
descendant of uranium just as radium is. Actinium,
however, does not fall in the main line of descent.
There is some probability that it marks the

100

RADIOACTIVITY

beginning of a collateral series, also springing from
uranium. The proportion of uranium which
disintegrates in this direction rather than towards
radium can only be very small.* It is possible that
uranium atoms, or those of one of its products, may
be of two different types, according to their internal
structure, and that on disruption each may give
rise to an independent chain of elements.

The heat output of radium and its relation to the
energy of the a-particle has already been men-
tioned. The first direct measurement of the heat
generated was made by Curie and Laborde. Using
suitably constructed calorimeters, they were able
to demonstrate a regular evolution of about 100
calories in an hour from a gram of radium — heat
enough to raise the temperature of an equal mass
of water from freezing to boiling-point. The
radium in these experiments was in radioactive
equilibrium with the emanation and radium A,
B and C, so that altogether four a-particles were
disengaged for each atom of radium which trans-
formed. The approximate correctness of this
first estimate will be seen from the following table,
in which the best results up to date are quoted :

1903. Curie and Laborde . ioo\

1904. Rutherford and Barnes . no
1904. Runge and Precht . . 105
1908. v. Schweidler and Hess . 118

gram
of radium

1909. Duane .... 120

1911. Duane . • . . 117

1912. St. Meyer and Hess . 132;

The correspondence of these resul^with the theo-

gram calories

emitted per

hour per

About 8%.

I 01

>U^^\

THE AGE OF THE EARTH

retical figure, 113 calories, which represents about
85 % of the energy,* is eminently satisfactory (see
Ap. A, p. 177).

The disintegration theory affords a simple
explanation of the origin of this energy. From the
new point of view the atom is no longer indivisible,
no longer the ultimate foundation of all matter ;
rather is it regarded as a tiny universe of electrons,
a whirling assemblage of charged particles. These
minute entities move with very high velocities
within the atom itself. In general the energy is
locked up, but in the radioactive elements the
atomic structure is unstable and from time to time
some of the energy can escape. The intrinsic
energy of the atom itself is the real source of the
heat liberated by radium, and the apparent
permanence of its output is simply an expression of
the immense supplies of energy which an atom
can store within itself.

The essential features of radioactivity are two :
the spontaneous emission of Becquerel rays, and the
atomic character of the change. The radiation
from a radioactive substance is proportional to the
quantity of the substance and to nothing else.
Temperature, being a function of molecular move-
ment, is without influence upon atomic trans-
formation. Experiments made at temperatures
ranging between that of liquid air and 1600 °C.
show that the same law of disintegration holds
throughout. Similarly, the effect of very high
pressures indicates that these inter-atomic pro-
cesses are quite independent of any external
influence. Whether in solution or in chemical

* Atomic recoil 6%, /3-rays4% 7-rays 5%, approximately.
102

RADIOACTIVITY

combination, whether taking part in energetic
chemical reactions or suffering bombardment by
its own radiations, the atom continues to obey
the law which determines its life period, unmoved
by any experiences through which we can oblige
it to pass. The radioactive properties are simply
superimposed upon the chemical and other
properties of the substance, and as far as laboratory
conditions are concerned, they are entirely spon-
taneous and can in no way be affected.

Whether or not radioactivity is a general
property of atomic matter cannot yet be an-
nounced. Potassium and rubidium are feebly radio-
active— a thousand times less than uranium — and
emit /8-rays. It has been thought that some other
metals may share in a minor degree the same
attributes, but up to the present no reliable results
have been forthcoming.

In its geological applications radioactivity
possesses a great interest. The most important
bearing of all lies in the part it plays as a source
of terrestrial heat. Radium is widespread amongst
the surface materials of the earth's crust. Rocks
and soils, and natural waters all contain radium
as a constituent, widely diffused but always
present. In the following two chapters the influence
of radio-thermal action in the earth and sun, and
on speculations as to their ages, will be dealt with.
Before passing on to these cosmic aspects of the
subject, a brief account may be given of the way
in which minute quantities of radium can be de-
tected and accurately measured — quantities which
in rocks average little more than a billionth of a
gram in each gram of the rock.
103

THE AGE OF THE EARTH

Radium itself is not directly dealt with ; it is
the emanation in equilibrium with it which is
actually measured. The emanation is separated
from a suitable amount of rock, mixed with air, and
introduced into an electroscope. After about
three hours, the emanation has generated radium
A, B and C in their equilibrium amounts, and the
emission of three sets of a-particles then steadily
proceeds. In the electroscope, the rate of fall of
the changed leaf depends directly on the con-
ductivity of the air or on the number of ions
which are utilised as carriers of electricity. Now
each a-particle, in virtue of its enormous energy, is
capable of producing in air about 100,000 ions, and
the total number present is therefore determined
almost wholly by the number of a-particles.
The P- and y-rays certainly have an ionising effect,
but so small that in this connection it may be
ignored. To be quite sure that all the ions are
taking an active part in the discharge and collapse
of the leaf, it is only necessary to observe the latter
before its divergence from the central support falls
to less than, say, 10°. The rate of collapse can
easily be gauged by observing the leaf through a
microscope with a divided scale attached to the
eyepiece, the time during which the leaf passes
over a given number of divisions being taken
with a stop watch. The rate of fall is propor-
tional to the conductivity, which depends in turn
on the number of a-particles expelled, and there-
fore on the quantity of emanation present in the
electroscope ; this quantity, provided that it was
initially in equilibrium with the radium in the
original rock, determines the amount of that

104

RADIOACTIVITY

element which was present. A constant can be
determined once for all by experiment with known
materials, which directly connects the rate of fall
in scale divisions per hour with the equivalent
amount of radium or uranium in grams. Some
idea of the delicacy of the method may be gained
from a" particular example. In the apparatus de-
vised by Prof. Strutt and used both in his own
investigations and those of the present writer, a
leakage of one scale division per hour corre-
sponds to 9 x io~13 grams of radium.

To use this apparatus it is necessary to bring the
rock into solution before the emanation can be
extracted. The solution is stored up for a few
weeks until the equilibrium amount has been
generated and the flask A containing it is then
attached to the water condenser B (Fig. 14). The
emanation can be expelled by vigorous boiling,
and greatly diluted with air, it passes out into the
gas-holder C. The stream condenses in B and
drops back. At the end of an hour the cooling
water is run out of B and the steam then drives
all the air charged with emanation into C. In
order that none should be allowed to escape back,
the connection at D is closed. Meanwhile the
electroscope F has been exhausted and the emana-
tion is made to pass into it through the tap at E,
the gas-holder being slowly filled with water. After
three hours the measurements may then be made.

Joly has recently varied this method in order
to save the labour attending the process of getting
a rock into clear solution. The mixture of finely-
powdered rock and fusion mixture is heated in an
electric tube-furnace, and the expelled gases

105

RADIOACTIVITY

containing the emanation are drawn straight
from the furnace. Carbon-dioxide is absorbed
by soda-lime, and the remaining gas is collected
and finally passed into the electroscope as before.
The absence or presence of thorium makes no
appreciable difference in these measurements, for
the life of its emanation is many thousands of
times less than that of radium-emanation. Joly
has utilised this fact in his solution-method for
determining minute quantities of thorium in
rocks and minerals — a method similar in principle
to that described above for radium.

The presence of radioactive elements in rocks
sometimes reveals itself in a most beautiful way.
In mica, cordierite, hornblende, chlorite, tourma-
line, and a few other minerals, small circular
spots known as pleochroic haloes are sometimes
seen. A tiny crystal can generally be detected
in the centre, usually of zircon, but sometimes
of apatite, epidote, rutile, or sphene. Until
quite lately the nature and origin of these intensely
pleochroic spots was entirely unknown. Joly
showed in 1907 that they are due to the radio-
activity of tiny inclusions around which they
spread spherically outwards. The a-rays dis-
charged from the minute central crystals are able
to ionise the biotite (or other mineral) and this
effect, to which the colouring action is due, spreads
just as far as the a-particle can penetrate. The
range of the different a-particles from the uranium
family is given overleaf, both for air and for biotite.

Now Bragg has shown that the ionising effect of
an a-particle is greatest just before it comes to
rest. We should therefore expect to find a number

107

THE AGE OF THE EARTH

of intensely coloured spherical shells existing
around the central crystal and corresponding in
each case to one or other of the range limits here

SOURCE OF CI-PARTICLE.

IN AIR.

IN BIOTITE.

Radium C ...

70-6

0-033

Radium A ...

48-3

0-023

Ra. Emanation .

42'3

O-O2O

Radium F ...

38-6

0-018

Radium ....

35'4

0-017

Ionium ....

28-0

0-013

Uranium i and 2

27-0

0-013

RANGE IN MILLIMETRES.

given. In section, and magnified 700 diameters,
the appearance of the shells would be as in the
following diagram.

FIG. 15.

Pleochroic Haloes in Biotite due to Uranium and its transformation
products. Magnified 700 diameters.

108

RADIOACTIVITY

The dimensions of the actual haloes are exactly
in agreement with the distances to which the
different a-particles can travel, and occasionally
the successive spheres of ionisation and colouring
are beautifully developed. In the biotite of Co.
Carlo w granite, Joly has found several very perfect
examples, and by his courtesy four of his photo-
graphs are reproduced in the Frontispiece. Not
only is the proof that the haloes are due to
a-rays conclusive beyond question, but the
correctness of Bragg's laws from which the
range in biotite is calculated are established in an
unexpected way. Thorium haloes are also found
(as in Fig. 2, upper left-hand part of the field), and
these again have diameters in perfect accord with
the demands of theory.

A point of interest in the history of igneous
rocks is that the haloes disappear when the rock is
heated. The presence of well-formed haloes,
therefore, implies the maintenance for very long
periods of fairly uniform conditions. So slowly do
these haloes form that it may become possible,
when they have been further investigated, to give
a rough minimum estimate of the age of the
minerals in which they occur. The radioactivity
of a zircon is much more intense than that of the
rock enclosing it, but the smallness of the quantities
involved is such that two or three weeks may
elapse between the expulsion of successive a-
particles or helium atoms. By the accumulative
effects of millions of these atoms a spherical halo,
faint in youth, but assuming a deeper tint with
age, is gradually produced.

109

CHAPTER VIII
THE THERMAL ENERGY OF THE SUN

The sun's heat and the conservation of energy — Mayer's
meteoric hypothesis — Helmholtz and contraction under
gravity — The earth's dependency upon solar radiation —
The work of Kelvin and Ritter — The insufficient contribu-
tions of atomic disintegration — Arrhenius' view of the
importance of molecular energy — Cyclic development of
the universe.

BEFORE the doctrine of the conservation of
energy was established, the steady radiation
of solar light and heat was not, in its quantitative
aspect, a phenomenon to be wondered at. Re-
garding the sun merely as a gigantic fire, philoso-
phers such as Leibnitz and Kant were satisfied
that the intense emission of energy was sustained
simply by combustion. As soon as the chemistry
of combustibles came to be studied, it was at once
evident that the energy derived from burning
alone would be hopelessly insufficient. If the
sun had been originally a colossal mass of the
most powerful explosives known to us, then,
under the most favourable conditions for main-
taining the present output of heat, all would have
been at an end within a few thousand years. The
total amount of available energy would not have
sufficed even for the historical period — a period
which is merely a ripple in the vast ocean of
geological time.

THERMAL ENERGY OF THE SUN

What, then, can be the source of the energy
which for millions of years has enabled the sun to
bathe the earth in a welcome glow of light and
heat ? How is the loss to be accounted for ? For
how long can the sun continue to radiate its
energy without becoming perceptibly colder ?
These critical questions must have appealed to
Mayer with some force when he recognised the
truth that energy could neither be created nor
destroyed. For the first time a sound explanation
became an imperative demand. Mayer realised
that in the collision and friction of bodies, heat
energy is acquired in strict equivalence to the
energy of motion which has apparently disappeared.
He saw that the mechanical generation of heat
would be of vastly greater importance in cosmic
evolution than the limited possibilities of com-
bustion. A piece of coal falling into the sun from
infinite space would yield, by the stoppage of its
motion, six thousand times as much heat as it
could provide by burning.

Applying these principles, Mayer thought that
the sun's heat might be traced to the kinetic
energy of swarms of meteorites. These bodies fall-
ing into the sun with enormous velocities, would
be competent, if only the supply were ample, to
generate the heat annually required. Kelvin at first
also advocated this view, but he was soon com-
pelled to relinquish it in favour of another ex-
planation less at variance with known facts. The
observations of astronomers were all against there
being a circulation and influx of meteorites to the
extent required by the sun if income and expendi-
ture of heat were to balance. Comets would suffer

THE AGE OF THE EARTH

resistance in their passage round the sun. The
rotational velocity of the sun would be constantly
impeded, and probably, far back in the past, it
would have been brought to a standstill. The
sun's mass would be appreciably increased every
year, and an immediate effect of this would be to
hurry up the earth in its orbit, so that each year
would be notably shorter than the preceding one.
Happily for the stability of the solar system,
there is no evidence for an infalling of meteorites
on the scale first contemplated.

In 1856, Helmholtz, another early worker in
the domain of energy, found a more satisfactory
escape from the dilemma. Instead of looking
outside the sun for the origin of the heat supply,
he sought for an internal source, and found one —
certainly a more efficient substitute — in the con-
traction of the sun's diameter under its own strong
gravitation. Knowing the amount of heat annually
radiated, it is easy to calculate that a shrinkage
of 1000 feet would make up the loss for five years.
The decrease in the apparent diameter would, at
this rate, never become detectable in human
experience. Helmholtz imagined a time when the
sun existed as a nebula spreading far out into
space. As it slowly cooled and contracted, the
mechanical work of shrinkage would reappear as
heat. Assuming the present sun to be a globe
of uniform density, Helmholtz calculated that its
past history must have been restricted to about
20 million years.

The annual output of heat had been determined
by Pouillet, and his result, which was too low, was
used by Helmholtz in this estimate. The solar

112

THERMAL ENERGY OF THE SUN

constant of radiation is measured by the heat in
calories, which would be absorbed in one minute
by a surface of one square centimetre placed
outside the earth's atmosphere at right angles to
the sun's rays. Allowance is made in this way for
the absorptive effects of gases and of the load of
dust held by the lower strata of the atmosphere.
Pouillet's value in these units was 176 and the
results of subsequent experiments, made up to
1905, varied between his figure and 4-1. This was
unsatisfactory, and, under the auspices of the
Smithsonian Institution, work has recently been
done to clear up the discrepancy. The constant
is now known with some certainty to be 1-95.
Using this figure the duration of the sun's heat
would, according to Helmholtz, be limited to 18
million years.

The surface temperature of the earth can owe
but little to its internal energy. Taking the
temperature gradient at i° C. in 32 metres, and
the average conductivity of rock as 0-004, the
temperature maintained by this flow of heat
alone would reach only 34° Absolute (239° C.
below zero). It is evident then that the genial
warmth of the greater part of the earth's surface
is maintained almost wholly by the absorption of
solar radiation. For this reason the active life
of our planet is intimately bound up with that
of the sun, and any age limit assigned to the latter
becomes a still more embarrassing restriction in
its application to the earth.

We may now return to our discussion of the
sun's vast expenditure of energy, armed with
data worthy of confident acceptance, and with the

i 113

THE AGE OF THE EARTH

knowledge that, for at least as long as the earth
has been a habitable globe, so long has the sun
emitted its life-giving rays at a rate not very
different from that of the present. The evidence
of geology is clear on this point. The geographical
distribution of plants and coral reefs in past ages
betrays no sign of a steadily cooling sun. In
some of the oldest sedimentary rocks which are
known, the imprints of raindrops have been found,
and the size and force of the latter were evidently
not very different from those which fall to-day by
the shores of seas and lakes. The intensity of
climatic forces has remained, on the average,
unchanged.

Kelvin somewhat mitigated the consequences
of Helmholtz's extreme view in his later treatment
of the problem. Helmholtz had assumed a sun
of uniform density ; but Kelvin pointed out that,
as the density probably increases enormously
towards the centre, the amount of heat which has
been already available may have been very much
greater than that previously calculated. Kelvin's
cautious spirit was not shared by his contem-
poraries, who readily accepted the smaller esti-
mates. The more daring investigations of Ritter,
and of other physicists who followed his lead,
did not support any period which exceeded 12
million years, Ritter showed that as the sun con-
tracted from the nebulous state its temperature
would at first begin to rise. Not only would con-
traction supply the energy necessary to sustain
radiation, but an even greater quantity of energy
would be available for heating purposes. An
interesting summary of Ritter's work will be

114

THERMAL ENERGY OF THE SUN

found in the second of the fascinating little
volumes by Arrhenius on The Life of the Universe.

Geologists found no consolation in these specula-
tive studies, and even from Kelvin's more liberal
allowance of time, an element of embarrassment
was not absent. While an annual shrinkage of
the sun's diameter by 200 feet would suffice for
the present, yet, unless at some time the sun's
temperature begins to fall, it is not clear why
shrinkage should continue. Cooling is none the
less certain because it is temporarily delayed, nor
because its rate is for a time diminished. Increas-
ing density would gradually put an end to effective
contraction, and the sun would then cool as a
white-hot ball would do — its capacity for replen-
ishing its losses having been exhausted for ever.
If this were all, then, in the course of a few more
million years, an icy death would overtake the
earth. In the last gleams of the fading solar
twilight our planet would disappear — a barren
and frozen world.

More recent views lead to a less pessimistic out-
look, and the twilight of the sun, though ultimately
inevitable, is removed to an indeterminately
remote future. Gravitation is manifestly an in-
sufficient cause to maintain the sun's heat for the
periods required. We need a supply not for less
than a dozen million years, nor even for the 100
million years which would have satisfied geologists
a decade ago. Some source a hundred times as
fruitful as contraction under gravity is required.
Happily there is no longer any need to regard the
sun as a serious difficulty, for there are locked
within its atoms and molecules stores of potential

THE AGE OF THE EARTH

energy capable of fulfilling every terrestrial
requirement.

The importance of radio-thermal action in the
sun was pointed out by Rutherford and Soddy in
1903. A month or two later, W. E. Wilson made a
calculation of the amount of radium which, if dis-
tributed throughout the sun, would entirely explain
the evolution of its radiant energy. He found
that 2*5 parts of radium in every million of the
sun's mass would be necessary. In uranium the
equilibrium amount of radium is only 0-34 part
in a million, so that, even if the sun consisted
entirely of uranium and its disintegration pro-
ducts, the heat generated would account for only
one-seventh of the total expenditure. Wilson
used in his calculation a solar constant of three
calories per minute, which is certainly too high.
His result must also be modified to allow for the
heating effects of the other radioactive bodies.
According to the most recent determinations, a
million grams of uranium would give out 77 calories
per hour, and in the same time a million grams of
thorium would give out one- third as much, each
element being in radioactive equilibrium. The
energy from the sun when divided throughout
its mass averages 300 calories per hour per cubic
metre. To sustain this steady evolution of heat
four million grams of uranium would be required.
The average weight of a cubic metre of solar
matter is only 1*44 million grams, so that by no
possibility could more than one-third of the sun's
heat be accounted for by radioactivity.

That radioactive bodies do exist in the sun
can admit of little doubt. Helium was first known

116

THERMAL ENERGY OF THE SUN

as a solar element and its abundance suggested
the presence of those radioactive elements from
which it could have been generated. Direct spectro-
scopic evidence has not yet revealed any traces of
radium, and, indeed, so minute are the quantities
involved that until recently there was little hope of
detecting it in this way. Uranium lines have
now been found in the sun's spectrum, and unless
the laws of radioactivity are totally different at
the temperature of the sun, we may safely
assume that uranium exists in equilibrium with its
associated elements. The sun's radiation is
destitute of the Becquerel rays, but this in no way
denies their emission from solar matter. Before
reaching the earth, the rays would be obliged to
pass through both solar and terrestrial atmo-
spheres, and the latter alone would be more than
sufficient to absorb them completely.

In the materials of the earth's crust, uranium
only averages about one part in 150,000, and
thorium one part in 50,000. If the proportions
of these elements which enter into the constitution
of the sun are of the same order, their contribu-
tions to the sun's energy can only be very small.
The importance of radio-thermal phenomena is
not felt until cooling has progressed to a more
advanced stage, as exemplified by the earth, when
the heat lost is balanced against that set free by
atomic disintegration. We must therefore find
some other means of escape from the embarrass-
ment of a rapidly cooling sun.

Arrhenius has made a bold attempt in this
direction. He calls to his aid a universal law,
first enunciated by Le Chatelier, which may be

117

THE AGE OF THE EARTH

stated as follows : // a system in equilibrium is
subjected to external influences which disturb its
equilibrium, then the internal reaction within the
system will be such as to oppose the external in-
fluences, i.e. the normal effects of the latter will
be partially overcome. This general statement
may be illustrated by the particular example to
which the chief appeal is made. In the chemical
changes which constitute combustion, heat is
evolved, and the reaction is said to be exothermic.
But the heat so liberated tends all the while to
prevent the reaction from proceeding, and were an
external source of heat of sufficient intensity
applied so as to raise the temperature, the com-
pounds previously formed would be again separ-
ated into their constituent elements. The
liberated elements possess a greater quantity of
intrinsic energy than when they are united to
form a compound. That is to say, not all the
heat supplied is able to exercise its normal effect
of raising temperature ; the internal reaction is
responsible for the withdrawal and absorption of
part of the energy. If now still more heat is
applied the same opposing tendency continues, and
the elements will again combine, this time forming
endothermic compounds characterised by further
absorption of heat, and consequently by higher
intrinsic energy than that which the free elements
possessed. The system shows a conservative dis-
inclination to be made hotter, and as more and
more heat is supplied, enormous quantities of
energy are accumulated in the recesses of the
molecule itself. In the case of water, the general
tendency is well illustrated. Ice at the freezing

118

THERMAL ENERGY OF THE SUN

point absorbs 80 calories and becomes water at
the same temperature. Water at boiling point
absorbs 540 calories and becomes steam. This in
turn, when raised to about 3000° C., dissociates
into hydrogen and oxygen, the absorption of
energy being 3800 calories. Although laboratory
conditions do not allow us to experiment further,
there are no grounds on which to suggest that this
is the end of the process.

While matter thus opposes an increasing tem-
perature by its internal reactions, it nevertheless
resists the reverse change quite as actively, and for
the same reason. If an intensely energetic exo-
thermic compound be allowed to cool, it will
strenuously refuse to do so at numerous stages.
The energy previously gained is emitted steadily
or explosively according to the thermal environ-
ment.

In discussing the constitution of the sun and
the source of its powerful radiation, this Law of
Reaction, as it is called, finds a pertinent applica-
tion. With its help Arrhenius has pointed out the
path which appears to lead us safely out of the
difficulty in which we were left by Helmholtz.
The chromosphere which, disregarding the mysteri-
ous corona, constitutes the outermost strata
of the solar atmosphere, is largely composed of
free elements at a temperature of 6000 °-70oo ° C.
Lower down in the photosphere, 9000° C. is prob-
ably attained. Temperature and pressure both
increase enormously with depth, and indeed, the
average solar temperature has been estimated at
a thousand times that of the chromosphere.
Under these conditions it seems reasonable to

119

THE AGE OF THE EARTH

suppose that the sun's interior is characterised by
compounds charged with a high concentration of
energy. Brought by convection currents towards
the surface such highly explosive compounds
would dissociate with expansion and an immense
evolution of heat. It may be to explosions of this
sort that the prominences are due. These vio-
lently projected gaseous tongues are shot out with
velocities which sometimes reach a thousand
times that of the swiftest rifle bullet. Since
energy is proportional to the square of the velocity,
it would appear from this that solar energies are at
least a million times greater than those of our
most powerful explosives. It has already been
stated that if the life of the sun depended solely
on the latter it would endure for only a few thou-
sand years. As it is, the energy seems amply
sufficient to last a million times as long. Here
then, furnishing a regular and sufficient income
from within, we have found an almost inexhaus-
tible source of heat, which is competent to main-
tain the sun's present expenditure for inconceiv-
atyy long ages, as most probably it already has
done in the past.

True, we are in the face of a new difficulty.
Whence arose this absorption and concentration
of energy in the first place ? It is evident that
once extinct, our sun could not be re-awakened to
the warmth of its former activity merely by
collision. Gravitational energy alone affords no
escape from the ultimate Warmetod, the thermal
extinction towards which the universe would
appear to be tending. If the development of the
universe be everywhere toward the equalisation

120

THERMAL ENERGY OF THE SUN

of temperature implied by the laws of thermo-
dynamics, the question arises — Why, in the abun-
dance of past time, has this melancholy state not
already overtaken us ? Either we must believe
in a definite beginning, in the creation of a uni-
verse furiously ablaze with energy, or else we must
assume that the phenomena which we have
studied simply reflect our limited experience.
Toward the latter alternative we readily incline,
the more so because of the hint it affords of cyclic
processes in the scheme of Nature. Not only is
energy being diffused ; somewhere, our hazy con-
ception tells us, energy is being elevated and
stored up. With profound insight, Spencer pointed
out in 1864 that it is to the attenuated nebulae
that we should look for the absorption and concen-
tration of energy. In the universe nothing is
lost, and perhaps its perfect mechanism is the
solitary and only possible example of perpetual
motion. In its cyclic development we may find
the secret of its eternity and discover that the
dismal theory of thermal extinction is, after all,
but a limited truth.

121

CHAPTER IX
THE THERMAL ENERGY OF THE EARTH

Temperature gradients, conductivity, and the rate at
which the earth loses energy — Kelvin's attempt to esti-
mate the age of the earth — King's treatment of the pro-
blem— Estimates by Becker and Suzuki — The distribution
of radium in rocks — The thermal equilibrium of the earth
-—Concentration of radium towards the surface — Con-
stitution of the earth.

AS the earth's crust is penetrated by bore-holes,
/~Y tunnels and mines, a steady increase of
temperature with depth is encountered. The
rate of increase varies greatly from place to place
as the following records show :

LOCALITY. DEPTH. TEMP. GRADIENT.

Anzin, France .

Wigan . . . . 750 metres

Sperenberg . . 1700

Mt. Cenis Tunnel 1600 ,,

Minas Geraes, Brazil

Calumet, Michigan .1430

0 C. in 15-3 metres
3°'° ».

, 36;5
', 86-0

, 12-20

Such a variation would naturally be expected
when the chemical reactions of weathering,
cementation and metamorphism are remembered,
and the unequal distribution of radioactive
elements in the earth's crust. The solution of
silicates by ground waters takes place with libera-
tion of heat, and it has been estimated that 120
calories are released when one gram of rock is

122

THERMAL ENERGY OF THE EARTH

decomposed by weathering. The processes of
metamorphism take place at the expense of the
earth's heat, and the net result of the complete
cycle in which (a) an igneous rock is eroded, (b)
the resulting sediments buried and transformed
into schists and (c) the latter brought again to the
surface by earth movement and denudation, is a
running down of energy involving a permanent
loss. Moreover, the earth loses heat from the
interior, not only by conduction, but .also by
convection. The circulation of ground waters
and the activities of vulcanism — \vhich include the
upward movement of molten magmas, heated
waters and gases — all result in the transference of
heat from the interior to the outer zones of the
crust. It is generally accepted, that when cooling
by convection is left out of account, the tempera-
ture gradient due to conduction alone is of the
order of I ° C. in 32 metres. If this estimate should
be in error, it is more likely to be too high than too
low.

To calculate the rate at which heat escapes from
the earth by conduction from the interior, it is
necessary to know the average conductivity of
rocks. This factor, in turn, varies greatly in
different materials, but in the case of the most
predominant rocks the conductivity is accurately
known, and the value k= 0-004 may be accepted
as very close to the true average value. The
variation of conductivity with increasing tempera-
ture and pressure scarcely affects the problem.
The former tends to diminish the conductivity, the
latter to augment it. As far as our present
knowledge goes, these two effects almost exactly

- 123

THE AGE OF THE EARTH

balance each other, and the assumption that
conductivity remains fairly constant with depth
is therefore justified. If r represent the earth's
radius, and dOjdr the temperature gradient, B being
the temperature, then the quantity of heat which
passes from the surface per second is given by
4irr2xkxdB/dr and can be readily calculated.

According to the Laplacian hypothesis, the
earth's original store of heat was derived from the
nebula from which it separated. Kelvin made the
assumption that as the molten globe cooled down
it was preserved by convection currents at a
temperature nearly uniform from centre to surface.
On attaining the point of solidification, it would
gradually become solid throughout, most probably
starting from the centre, and only when solidifica-
tion was complete could the surface continue to
cool further. The problem which Kelvin set him-
self to solve was this : Given a solid globe originally
at a uniform temperature of 7000° F. (3871° C.),
and subsequently cooling down, to calculate the
time which would be required for the establish-
ment of the present surface gradient.

Taking the most probable values for conductivity
k, density of rock />, specific heat of rock o-, and
temperature gradient d&jdr, he applied his data to
Fourier's differential equation for the linear con-
duction of heat —

dB=k^ d*8
dt pa- ' dr*

Solving this equation for the unknown factor /,
the required time period was found. The high
initial temperature was chosen from very meagre

124

THERMAL ENERGY OF THE EARTH

data, as representing a maximum figure for the
melting-point of rock. Kelvin was particularly
anxious that his treatment should provide an over-
rather than an under - estimate of time. The ex-
periments of Dr. C. Barus have shown that diabase,
a good typical rock, becomes thoroughly liquid at
1200 °C. If Kelvin had used this temperature
instead of his arbitrary 3871° C., his periods of
cooling would have been reduced to less than one-
tenth of those actually arrived at.

For enormous periods of time, the development
of a temperature gradient would be restricted to
the earth's outer zones. The interior, in complete
thermal isolation, would remain unaffected, its
loss of heat being quite insensible. The limited
thickness of the outer shell, in which cooling would
make itself felt, is made clear by the following
figures :

160 miles in 100 million years.

240 ,, ,, 240

320 „ „ 600 „

570 ,, ,, 1000 ,,

The different periods which Kelvin favoured in his
famous contributions to this problem have already
been mentioned, and are tabulated below.

1862. 96 million years (limits 20-400).
1876. 50 - 90 „

1897. 20 - 40

Clarence King, in 1893, applied a new criterion
to the subject, taking into consideration the effect
of pressure in raising the melting-point of rocks,
and the necessity for an earth which should be
stable under the influence of tidal stresses. Barus

125

THE AGE OF THE EARTH

had measured the melting-point of diabase at
various pressures, and a law of variation of
melting-point with depth was formulated on his
experimental results. If this law were to hold as
far as the centre, diabase would there be able to
exist in the solid state at any temperature below
76,000° C. Thus the hypothesis arose that solidi-
fication would begin at the centre owing to the
high pressure obtaining there. Under these
conditions, however, a temperature gradient was
already developed. If the gradient were to exceed
the rate by which the fusion point of rocks is
raised by pressure, the former would catch up, and
at a certain depth the temperature would reach
the fusion point and a zone of fluid rock would be
inevitable.

King accepted diabase as a representative rock,
and rejected any distribution of heat which would
demand a liquid zone in that part of the earth's
crust where diabase or similar rocks would be
expected to prevail. This procedure is justified by
the consideration that were such a zone to exist,
the earth would be incapable of maintain-
ing tidal stability, and the crust would break
down. King found that in the admissible cases,
the initial temperature of crustal solidification
would not exceed 2000 °C., and that the period
of cooling, which would reduce the gradient to that
of the present was limited by 24 million years.
Higher initial temperature would involve fluidity,
and superior age necessitate a lower surface
gradient. The following curves represent the
gradients of Kelvin's earth of 100 million years,
and King's earth of 20 million years, in relation

126

THERMAL ENERGY OF THE EARTH

to the diabase fusion-point curve It will be seen
that according to King's argument, Kelvin's earth
implies the impossible condition of a liqud zone
from A to B.

In 1910, Becker attempted to deal with the
same problem without relying on the tempera-
ture gradient, it being considered that owing to

TEMPERATURE

KX) MILLION YEARS

Zoco'C

xoqo

10GO*

DtPTH FRO\ SURFACE.

FIG. 16.
Temperature Gradients in relation to the Fusion point of Diabase.

the presence of radium in rocks the gradient could
not be trusted. In its place he assumes that the
crustal strains associated with upheaval and sub-
sidence are completely relieved at the surface of
easiest fusion — and that according to the calcula-
tions of Hay ford on isostatic compensation, the
present depth of that surface is 71 miles. At that
depth, therefore, the temperature curve and the
diabase curve approach most closely, so that the
additional temperature required to produce fusion

127

THE AGE OF THE EARTH

and relief of strain there becomes a minimum. In
the above diagram, C would represent the point of
easiest fusion. Becker justifies his choice of
diabase by showing that on the Laplacian law of
density, rocks of this type become predominant
at depths greater than 40 miles, the more acid
rocks, lying above, being more refractory. Tidal
stability is provided for by rejecting any tempera-
ture curve which crosses the diabase line in the
zone of basic rocks. The most probable earth,
according to Becker, is one with an initial tempera-
ture of 1300 °C., which would attain a surface
gradient of I ° C. in 42 metres in 60 million years.
He concludes that only " a tenth of the heat
emitted by the earth can be ascribed to radio-
activity plus all other exothermic chemical
transformations. ' '

A Japanese estimate of the time elapsed since
the molten surface of the earth began to solidify
appeared in 1912. Suzuki makes the assumption
that a thin solid crust has gradually increased in
thickness, so that the latent heat of fusion liberated
at the junction of solid and liquid rock is equal to
the heat lost at the surface. The present thickness
of the crust is assumed, on the authority of Milne
and Arrhenius to lie between 30 and 40 miles.
Granting these postulates, the thermal constants
for basalt and granite lead to an age of 20 to 60
million years according to the thickness of the
crust and the material of which it is composed.

It is surprising that Becker and Suzuki should
have treated the problem in this restricted way.
The heat evolution attending atomic disintegra-
tion was established in 1903, and in dealing with

128

THERMAL ENERGY OF THE EARTH

the earth this phenomenon must be regarded as one
of fundamental importance. To ignore the signifi-
cance of radio-thermal energy is to reduce the
problem to a mathematical exercise, interesting,
no doubt, but with little value in its geological
application. Let us make a simple calculation
of the quantity of radium, which, if uniformly
distributed throughout the earth, would make
good the loss of heat. If Q is the heat generated
per second by the radium in each cubic centimetre,
then we have

whence, Q=6x io-16 calories per second.
=2-16 x IQ-11 calories per hour.

Now i gram of radium in complete radioactive
equilibrium emits 216 calories per hour and
consequently all the heat would be supplied by
I0-i3 grams per cubic centimetre, or i-Sxicr14
grams per gram of earth material.

Turning to the rocks themselves, the actual
amount of radium is found to be a hundred times
more than we want. Strutt was the first to
discover this embarrassing richness, and his
results, with those of later investigators, are
summarised in the adjoining table. Joly's results
are given apart from those of other observers, for
they were arrived at by the fusion method, and,
moreover, separate rocks were not examined. A
composite mixture of typical rock specimens was
made up and a single analysis then sufficed
to determine the average radium content. It will
be noticed that Joly's results are consistently
higher than those found by the solution method.
K 129

THE AGE OF THE EARTH

v0 i i i

O i

* 1 1 1

2- I

o* ^t* ^o w

•O

O 00

iO *O t^» t^*

0 O O O

O H

ON vO ^- iO ^t"

00 H

C4 CO

iO 00 Tt- »O

00 t^*
M 10

MM H, 0

•H N

Tj- -4- VO O

CO 00

ON fO w <O

CO M

ir^ 00 vO 00

vO O

(S H N O

N fO

H CO 00 -<4-

rf vO

ro oo

•

• •

• J2

•

(*|

5 b ii

I ' J ... I i

fi ,« j? il

w h W fe

Holmes

CJ
rt E^>

130

THERMAL ENERGY OF THE EARTH

Joly claims that his own procedure is more reliable
than that followed in the solution method. Up to
1909, Joly had himself employed the latter method
in the examination of 126 igneous rocks. In
striking disagreement with the results of other
workers, he found an average radium content of
7 X io~12 grams per gram of rock. An explanation
of the discrepancy is not yet forthcoming, but in
the light of his most recent work, which gives an
average of 2-5 XiO'12, he has now suggested that
his earlier results be set aside. Measurements of
thorium in rocks are not yet so plentiful as those
of radium, and most of our present knowledge of the
distribution of this element is due to Joly and
Fletcher. The most probable averages of the data
now available may be summarised as follows :

TYPE OF ROCK.

AVERAGE PER GRAM OF ROCK.

RADIUM.

THORIUM.

Igneous
Sedimentary
Metamorphic
Deep-sea deposits

2-5 X io-12 grs.
1-5 Xio-12 „

2-0 XIO-12 ,,

5-0 xio-12 „

2-0 x io-5grs.
i-oxio-5 ,,
I-5X io-5 „

Accepting these figures for igneous rocks
provisionally and combining them with the
respective heat emission of radium and thorium
in complete radioactive equilibrium, viz.,

Radium* per gram 6 x io-2 calories per second.
Thorium „ „ 7'5Xi<r9

* Equivalent to 3,000,000 grs. of uranium.

THE AGE OF THE EARTH

it is clear that each gram of the earth's crust is a
source of heat supplying on an average i5Xio~14
calories per second on account of its radium
content, and 15 Xicr14 calories on account of its
thorium content. The total heat emission is
therefore of the same order in each case, and
amounts altogether to 30Xi<r14 calories per
second. The whole mass of the earth is 6xio27
grams, and if this were the source of as much
radio-thermal energy throughout, the supply of
heat in 1000 million years would have been suffi-
cient to raise its temperature to about 40,000° C.,
and the present gradient should be many times
greater than it is. This conclusion cannot be
reconciled with the evidence afforded by the
crustal rocks, both their structure and temperature
gradients being decisively against any such
possibility.

There are three cases which may be considered.
The earth may be in thermal equilibrium, gaining
as much heat as it loses and cooling only as the
slow decay of the radio-elements permits ; or it
may be growing hotter, or, which is very unlikely,
it may be cooling more rapidly than it would do if
in radio- thermal equilibrium. The first case is
the one now regarded with most favour. If the
earth has cooled at all, and there seems to be no
sound reason why we should altogether abandon
that venerable conception, it must at some time
have attained a condition of equilibrium. With
the slow march of atomic disintegration its own
rate of cooling would then keep time. The tempera-
ture gradient would be maintained solely by radio-
activity for an immeasurably long period.

132

THERMAL ENERGY OF THE EARTH

The superabundance of radium which seemed
to be implied by Strutt's original work is certainly,
as he then suggested, restricted to the surface
rocks. The interior of the earth must be relatively
free from radium and thorium. It is easy to
calculate the thickness of the outer zone of the
earth's crust, which would suffice to supply the
stream of heat passing to the surface. The
temperature 6r at any distance Y from the surface
is given by the following equation, where k is
conductivity, h the heat production of radium and
thorium per second, in each gram of rock, p the
density of the rock, and D the total depth of the
radioactive layer :

At the base r becomes equal to D and the tempera-
ture 0D is therefore given by

Using the figures given above, the thickness of the
radioactive layer would be restricted to about ten
miles and the basal temperature would reach
only 250° C. This result cannot be held to
express the facts, for there can be no doubt that
the radium and thorium content decreases with
depth for the same reason that the type of rock
varies with depth. A glance at the table on p. 130
will show that there is a rough proportionality
between the acidity or percentage of silica of a
rock and its radium content. The more basic

133

THE AGE OF THE EARTH

rocks are much poorer in radium, and, as would
be expected, in thorium also. Now we have good
reason to suppose that the more deep-seated rocks
of the earth's crust are of basic and ultra-basic
composition, and that below the 30 -mile crustal
zone they are exclusively ultra - basic, perhaps
similar in composition to the material of stony
meteorites, with which they agree almost exactly
in density (3-4). This information, which it might
be thought would be for ever withheld from us,
is derived from the study of earthquake waves.
The latter in passing through the earth's interior
carry with them a record of the type of material
they have penetrated. Within the stony zone, which
extends downwards for several hundred miles, and
separated from it somewhat sharply, lies the heavy
core of the earth (density about 7-8), probably
of metallic composition, like the iron meteorites.
If we may judge from the latter, this nucleus is
entirely free from radium, and that there is safety
in this analogy is indicated by the very low
radium content of such native iron as has found
its way in basaltic magmas up to the surface.

We have already seen (p. 30) that a first
differentiation of the original heterogeneous
material from which the earth was built would
result in the formation of a metallic core surrounded
by a stony zone. The further differentiation of the
latter, whereby the crust with its abundant variety
of acid and basic rocks was developed, is of too
complex a nature to be considered here. It is
very probable that as the more siliceous con-
stituents separated and became concentrated
towards the surface, they carried with them their

134

THERMAL ENERGY OF THE EARTH

store of radio -elements. In this way, basing our
ideas on evidence quite independent of the
temperature gradient, we are led to the remarkable
conclusion that the radium and thorium of the
earth are to be found almost exclusively in the
earth's crust. The most probable depth of the
radioactive layer may therefore be placed^ at 30
miles and the basal temperature in this case would
be about 750° C., which would be more in
accordance with the requirements of volcanic
phenomena. Moreover, it must not be forgotten
that the heat lost by the upward movement and
convection currents of rock magmas, heated
waters and gases, has also to be accounted for.
The radio-thermal equivalent must be substantially
increased to include this phase of the subject.
The basal temperature of 750° C. is only a
minimum, and the higher temperatures demanded
by geology are not therefore inconsistent with the
facts. However, until more data are accumulated,
it would be rash to attempt to deduce the exact
distribution of the radio-elements in the crust,
but already we may assert with confidence that
the crustal average is somewhat lower than that of
the surface rocks in which granitic types form so
large a proportion. What the average actually
may be cannot yet be decided.

Kelvin's problem must now be reversed. It is im-
possible to deduce the earth's age from its thermal
condition. We can only say that the age must be
very much greater than Kelvin calculated. The
new problem which presents itself is to determine
the thermal history of the earth, accepting its
antiquity as a known or partially known factor.

THE AGE OF THE EARTH

The science of radioactivity is a welcome addition
to the tools which the geologist employs in his
difficult task of elucidating the earth's history, and
it is peculiarly valuable in helping him just where
he has hitherto had most cause for despair.

136

CHAPTER X
RADIOACTIVE MINERALS AND THEIR AGES

The rate of helium production from uranium and
thorium — Lead the final product of the uranium family —
Its accumulation in geological time — Lead and helium
ratios as a measure of geological time — Assumptions to be
granted — Value of analyses in deciding quality of material
for estimating ratios — Necessity for fresh, stable, primary
rock minerals — Strutt's work on the helium ratio — The
lead ratio — Boltwood's collection of analyses — Examples,
with geological data — The author's work on the Devonian
minerals of Norway.

AS we saw in Chapter VII the a-particles which
,/"\. are emitted at certain points in the line of
descent of the radioactive elements have been
identified with helium. Fortunately the evidence
is conclusive, for upon this identification depends
the latest and most elegant method yet devised of
measuring geological time.

When all the members of a genetically related
series of radio-elements are in equilibrium, the
transformation proceeds in such a way that an
equal number of atoms of each element disinteg-
rates in the same time. When an atom of uranium
disintegrates it does so with the production of
two atoms of helium. At the same time, as indi-
cated in the diagram, page 190, each of six other
members of the family also emit a single atom of
helium. Consequently, the total number of helium

i37

THE AGE OF THE EARTH

atoms which are liberated in the course of the
complete transformation of a single atom of
uranium is eight. From this result and the re-
markable measurement made by Rutherford and
Geiger — that 3-4 xio10 a-particles or atoms of
helium are expelled per second from a gram of
radium — it is possible to calculate that the annual
production of helium from a gram of uranium in
equilibrium with all the other products of the
series is 10-7 Xicr8 cubic centimetres. Measure-
ments of the ionising power of thorium and its
chain of dependent elements may be utilised to
calculate the same rate in the case of the thorium
family. It is found that one gram of thorium is
equivalent in helium generation to 0-26 gram of
uranium. The details of these calculations will be
found in Appendix A.

While there is little uncertainty in these indirect
results, it is gratifying to know that Prof. Strutt,
during 1909-10, verified them both by a direct
appeal to experiment. In certain minerals which
contain radioactive constituents, the evolution
and accumulation of helium must have been
steadily proceeding for very long periods. Before
a direct determination of the rate of helium pro-
duction can be made, it is evidently necessary
to use material entirely free from that element.
Strutt worked with richly radioactive minerals,
from large quantities of which he expelled com-
pletely the accumulated store of helium. This was
done by preparing solutions with every precaution
to avoid the presence of undissolved particles of
mineral, and afterwards boiling them till the helium
was removed. The solutions were then put aside

138

RADIOACTIVE MINERALS

until a fresh supply of helium had been generated
in sufficient quantity to be detected and measured.
To isolate the gas from the solutions and accu-
rately to determine its volume was obviously a
matter of great experimental difficulty, so minute
were the volumes dealt with. However, as the
result of the exquisite delicacy of his methods,
Strutt brought his experiments to a successful
issue.

The minerals used were pitchblende and thori-
anite, the former containing uranium alone, and
the latter both uranium and thorium. The
annual production of helium per gram of the
parent element (in radioactive equilibrium with
its respective family) was found to be:

(a) in the case of uranium :
IO-6XIQ-8 ccs. or I-88XIQ-11 grams.
107x10"' ccs. was the calculated estimate.

I cc. would therefore be formed in 9,600,000 years.

(b) in the case of thorium :
2-4x10-" ccs.

i gram of thorium is therefore equivalent in its
rate of helium generation to 0-23 grams of uranium,
the calculated estimate being 0-26 grams.

The remarkable concordance of these results
with the theoretical requirements is an eloquent
tribute to the refined methods and experimental
skill with which the measurements were carried
out.

Experiments carried out by Boltwood and
Rutherford in 1911 afford an equally striking
confirmation of the conclusions on which the
theoretical results were based. They measured

139

THE AGE OF THE EARTH

the rate of production of helium from radium, the
latter being in equilibrium with its early disin-
tegration products, three of which also emit a-par-
ticles. Radio-lead and polonium were completely
removed. The annual evolution of helium to be
expected was 158 cubic millimetres. The first
direct determinations were made by Sir James
Dewar in 1908, and his best results corresponded
to 169 cubic millimetres. Boltwood and Ruther-
ford arrived at a much closer agreement, their
figure being 156 cubic irdllimetres.

These results cannot fail to inspire the convic-
tion that our atomic theory of matter is essentially
correct. We are in possession of two experimental
facts. The number of helium atoms expelled per
second from a gram of radium has been directly
counted, and the volume of helium accumulated
in a year has been directly measured. The number
of atoms in a given volume of helium (at N.P.T.)
can be deduced at once, and the calculation is
independent of any underlying theory. Calculation
gives 2-69XI019 atoms per cubic centimetre;
the atomic theory demands 272Xiols.

Although there can now be no doubt that
helium is one of the stable disintegration products,
yet there is no direct evidence as to the identity
of the ultimate products in the direct line of
descent. In the uranium series indirect evidence
points to lead with a considerable degree of cer-
tainty, but the end product of the thorium family
is still unrecognised. In every uranium-bearing
mineral the parent element slowly breaks down,
while the final product of the transformation
accumulates at its expense. Hence, if lead is the
140

RADIOACTIVE MINERALS

favoured element it ought to be found in associa-
tion with uranium in all minerals which contain
the latter. Moreover, in minerals which can be
proved to be of the same antiquity, the amount
of lead per gram of uranium should be constant ;
further, in minerals of various geological ages the
proportion of lead should vary according to the
latter. A mineral which began its accumula-
tion of lead in pre-Cambrian times should cer-
tainly contain more at the present time than one
in which lead has been collecting only since, say,
the Tertiary outburst of igneous activity. The
same statements apply equally well to the case of
helium.

In so far as these principles may be used con-
versely to test the identity of lead with the ulti-
mate product, they lend every support to that
important conclusion. Dr. Hillebrand, the leading
authority on the analysis of uranium-bearing
minerals, has never in the course of a long experi-
ence found uranium unaccompanied by lead. It
was this constant association which led Boltwood,
in 1905, to suggest the probability of a genetic
relationship existing between these two elements.
In 1907, Boltwood went farther and showed that
for minerals of the same age the amount of lead
for each gram of uranium, or the ratio Pb/U, was, in
general, nearly constant. He collected all the
best analyses of primary uranium minerals, but
unfortunately he omitted to give the geological
details of their occurrence. As will be seen in the
present chapter, when the relative ages of the
minerals are compared with their lead ratios, a
striking proportionality discloses itself.

THE AGE OF THE EARTH

The evidence of atomic weights is also favour-
able to lead. The complete disintegration of an
original atom of uranium may be expressed as
follows :

U -> 3He + Ra -> 8He + Pb
238-5 ^3*994 226-36 3-994 207-08

238-5 238*^34"" ~~239'03

Atomic weights are appended to each symbol,
and the totals, which should be equal, are
placed underneath. The agreement is close, but
not as convincing as one could desire. Two alter-
native explanations of the discrepancies are sug-
gested. Either lead is not the final product, or
the atomic weights of both radium and uranium
are too low by about 0-5.- Neither alternative
can readily be granted, but it may be pointed out
that since uranium is the heaviest known element
and radium follows not far behind, any impurities
whatever, with the exception of thorium, would
have the effect of lowering the observed atomic
weights.

Accepting the above equation as substantially
correct, the mass of lead generated in one year from
a gram of uranium can now easily be calculated.
For eight atoms of helium, one of lead is produced,
or, mass for mass, six and a half times as much.

Consequently, as a gram of uranium involves the
annual production of i-88x IQ-11 grams of helium,
the associated lead which remains must amount
to i-22 = io~10 grams. If this rate were constant
we could find how long it would take for any mass
of uranium to become completely converted into
142

RADIOACTIVE MINERALS

helium and lead. However, the rate is not con-
stant, but is proportional at every moment to the
quantity of uranium remaining unchanged. As
the parent element becomes exhausted, it dis-
integrates more and more slowly.

Now in very considerable periods amounting to
hundreds of millions of years only a very small frac-
tion of the uranium originally in existence is de-
cayed. For this reason, if only a small proportion
of lead or helium has collected in a mineral since it
began its life-history, then no serious error will be
made in assuming their rates of evolution to have
been constant. In minerals which have been in exist-
ence for 400 million years the slowing down is
only about 5 %. If an appreciable error should
arise in ignoring this decline, then in place of the
present-day percentage of uranium the time-
average must be substituted (see Ap. A, p. 179) ;
that is to say, the amount of uranium which, if it
did break up at a regular rate, would evolve the
same quantities of the ultimate products.

Having calculated this uranium average, Um,
with the necessary approximation, the total
quantity of lead or helium accumulated in a
mineral would then provide a direct measure of
its age.

Using lead as the age-index, and knowing its
percentage, Pbt, in the mineral, then the time it has
taken to collect, i.e. the age of the mineral, Pbt, is
given by Pbt/Um X 8200 million years.

Using helium as the age-index, both thorium
and uranium must be estimated. The amount of
thorium may be conveniently expressed in terms
of uranium, since the latter is four times as active

143

THE AGE OF THE EARTH

in its helium production as thorium. The total
equivalent quantity of uranium, Ue, is thus known,
and thorium then need play no further part in
the calculations. It is found most convenient to
measure the amount of helium, Het, as the volume
in cubic centimetres per gram of mineral. In this
notation the age is given by Het/Ue X 9*6 million
years.

The validity of this procedure evidently demands
the granting of certain obvious assumptions. Our
choice of suitable minerals will not only be limited
by these considerations, but the reason for a
particular choice will be justified. The assump-
tions fall under the following headings :

(a) That no appreciable amount of lead or

helium was present at the genesis of the
mineral.

(b) That no lead, helium, or uranium has sub-

sequently been added or removed by ex-
ternal agencies.

any other radioactive process than those
already suggested.

The first and second suppositions bring up the
whole problem of the origin of minerals. It is
possible, by means of a physical examination, to
decide whether a mineral is of the same age as the
rock in which it occurs, or whether it is older or
younger. In igneous rocks, such as a granite, the
majority of the minerals are of the same antiquity
as the rock, that is, they date from the period of
consolidation of the rock magma. The component
mineral particles of most sedimentary and detrital

144

RADIOACTIVE MINERALS

rocks existed long before the strata were laid
down, whereas the cementing materials by which
they are consolidated are partly furnished by
percolating solutions and therefore may be sub-
sequent to the period of deposition. In the same
category come those ore deposits which occupy
the fissures and crevices of pre-existing forma-
tions.

In whatever way a mineral may occur, its history
can always be traced back either directly or by con-
jecture to an igneous rock, and it is rarely that it
is possible to go beyond the magma from which
such a rock must have consolidated. And even
if this can be done in exceptional cases, there lies
behind still another magma to which the material
can be referred. Consequently, the minerals of
igneous rocks are regarded as primary or original,
and here we may briefly consider a few facts
relative to their crystallisation from a molten
condition.

A molten rock is regarded as a solution in
which the numerous constituents are dissolved
one in another. Now certain of th^jconstituents
can only remain in solution provided the latter is,
with regard to them, very dilute. That is to say,
they are only slightly soluble in a solvent com-
posed of the rest of the rock material. Conse-
quently, substances of which these constituents
form an essential part will, as a general rule, be
the first to crystallise ; as examples, zircon,
sphene, and apatite may be cited. For the same
reason it happens that the magma does not
remain homogeneous, but rejects certain of the
rarer elements which collect together in a sub-

L 145

THE AGE OF THE EARTH

sidiary magma of peculiar composition. To this
concentrate of exceptional constituents, the gases
and water vapour expelled during solidification
are also added, and serve to maintain it in a state
of aequo-igneous fusion, even when the bulk of
the rock has already crystallised. The residual
liquors yield the minerals of pegmatites and of
drusy cavities. Certain minerals which are con-
spicuously rare in the body of the normal rock, are
often developed on a large scale in pegmatite
dykes, and it is from these that the most perfect
and beautiful crystal forms are generally obtained.

Amongst the elements of limited solubility in a
rock magma, uranium and thorium must be
placed. The accessory minerals of ordinary
igneous rocks, such as those already mentioned —
zircon, sphene, and apatite — are rich in the radio-
active elements when compared with commoner
minerals like felspar and hornblende. In general,
the richness of a mineral in uranium seems to
depend on its position in the order of consolidation.
The minerals first to be formed claim the greater
part of the available store. The original surplus,
unable to dissolve in the magma, is held over,
and, should it be rich in radioactive ingredients,
uranium- and thorium-bearing minerals may be
formed during the later stage of pegmatitic in-
trusions. It is a striking fact that, as primary
constituents, these minerals invariably occur in
pegmatites associated with granite or syenite. As
examples, pitchblende or uraninite, thorite, thori-
anite, and monazite may be mentioned.

We must now consider the part played by lead
and helium during the genesis of minerals. Before
146

RADIOACTIVE MINERALS

the consolidation of the magma, both these elements
must, of course, have been generated within it for
an unknown period. As to the effect of physical
conditions upon radioactive transformations it has
already been shown (p. 102) that all the evidence
points to the conclusion that these atomic changes
are independent of the temperatures and pressures
under which a molten magma exists. The helium
already present at the time of crystallisation
appears to behave physically in no way different
.from the other gases. There is no evidence that
it tends to congregate in any particular mineral.
A small proportion may be distributed through
the resulting rock, but probably the larger share
is expelled.

The lead which may be originally present fol-
lows a similar course. The metal is rejected,
not only by the primary magma but, with rare
exceptions, by the residual magma also. It
finds no definite place in igneous rocks. Doubt-
less a certain amount of lead is retained in
the molecular network of crystals, but that
amount is not high. In the rocks of Leadville,
Colorado, Hillebrand found an average of less
than 0-002 per cent of lead. In the nepheline
syenite of Southern Norway, using specimens
free from minerals which one would expect
to be comparatively rich in accumulated lead,
the present writer was able to determine a per-
centage of only 0-0004. If> then, there should
be initially a greater quantity of original lead,
where are we to look for it ? Probably the most
of it goes to form lead-ores, such as galena. Separ-
ated from the pegmatites it appears in the later

THE AGE OF THE EARTH

phases of ore deposition which follow on the heels
of igneous activity. In company with hot gases,
sulphide solutions, and a number of metallic
companions, our lead is carried away and deposited
in the fissures encountered by the mineralised
waters. If the agency of magmatic gases appears
to have been an important factor in the produc-
tion of ore bodies, the origin of the latter is said
to be pneumatolytic. Brogger has shown that in
Southern Norway galena was one of the minerals
to be formed in this way.

In the last phase of this complex series of
operations, magmatic waters contribute their
share to the filling of mineral veins, and it is
amongst these hydatogenetic ores that galena is
most usually found. The important point is that
lead, for the most part, is drawn from the primary
magma at, or perhaps before, the time of crystallisa-
tion, and it is not until the igneous activities have
declined that it again appears in an active role.

Let us now consider the effect of the original
distribution of lead and helium in a newly-formed
rock. It will be clear that an analysis of the rock
as a whole would give values of Pb/U and of He/U
much higher than those corresponding to the
period since consolidation. Of the total amounts
of lead and helium, part would be originally
segregated in the rock and part would be due to
subsequent genesis. In most rocks, the former
part is of sufficient magnitude altogether to in-
validate the use of the ratios as age indices.
This difficulty can be avoided by confining atten-
tion to particular minerals — indeed, to just those
minerals which concentrate within themselves

148

RADIOACTIVE MINERALS

the radioactive parent elements. Within them
lead and helium may accumulate to such a degree
that the amount initially present becomes negli-
gible. Zircon from the Devonian syenites of
Southern Norway contains more than twenty
times as much lead as the rock in which it occurs.
Roughly, we may say that since the zircon came
into being its content of lead has multiplied
twenty times. Thorite may accumulate a hundred
or a thousand times as much lead as it possessed
at first. For the same reason, minerals like zircon
and sphene often contain hundreds of times as
much helium as the rock from which they are
taken, and there is little possibility of error in
assuming that they have themselves generated
the whole of their supply.

Another difficulty, and a more serious one, must
now be faced. Can we be sure that for periods of
hundreds of thousands years a mineral has remained
comparatively unaltered by external agencies?
With regard to helium there is undoubtedly a
tendency to escape. Strutt has demonstrated that
when a radioactive mineral has been powdered,
helium begins to leak, rapidly at first, then at a
diminishing rate. Even crystals washed out of
their original matrix showed a considerable leak-
age of helium. The observed rate of escape
always exceeds the rate of generation, and it
therefore follows that during the life-history of a
mineral the conditions must be specially favourable
to the retention of helium, for otherwise the latter
could not have accumulated. Nevertheless, these
experiments prove conclusively that the majority
of minerals do not contain their full store of

149

THE AGE OF THE EARTH

helium, and it is a matter for surprise that they
contain so much. Consequently, ages deduced
from the helium content of minerals can be
regarded only as a fraction of the true age.

Dealing with lead and uranium, we must con-
sider the tendency to alteration of the minerals
in which they occur. From the surface down to
the permanent level of the ground waters, rock
material is subject to weathering. The more
soluble constituents are leached out and com-
plex silicate minerals are decomposed by the
combined action of water, oxygen, and carbon-
dioxide. It is in this belt of weathering that
igneous rocks suffer most change. Many of the
primary minerals are broken down and alteration
products take their place. All the reactions
involve considerable increase in volume, and not
only are minerals altered in place, but material is
carried away and deposited elsewhere as secondary
minerals. Can we be sure that lead and uranium
have remained untouched during this redistribu-
tion ? In some cases we cannot, but fortunately
for our purpose many of the most valuable minerals,
like zircon, are dense and exceptionally stable.
A mineral is only stable over a limited range of
conditions. It adapts itself more or less readily
to its physical environment. Certain minerals,
however, are much more capable than others of
withstanding great changes without undergoing
metamorphism or alteration, and amongst these
are many of the uranium-bearing minerals.

When there has been a migration of lead or
uranium, an appeal to analysis will rarely fail to
dispel the difficulty by disclosing the fact. It is

150

RADIOACTIVE MINERALS

inconsistent with the chemical properties of these
elements that both should have been affected in
the same proportion, and hence the ratio of lead
to uranium obtained from different minerals of
the same geological age affords an immediate test
of the extent to which they have suffered from
alteration in the course of their history. If the
analyses give consistent results, it can be safely
assumed that the effects of alteration have been
inconsiderable ; if there are marked discrepancies
the results must be rejected as valueless from a
chronological point of view. A microscopical ex-
amination of the minerals before analysis is a useful
safeguard, for in this way altered material can
often be detected. It is clear that reliable conclu-
sions can only be drawn from minerals which are
undoubtedly fresh.

Becker has criticised the method by directing
attention to a suite of minerals from Llano Co.,
Texas. Their geological age is well defined. The
Burnet granites with which they are associated
are intrusive into a series of schists and quartzites,
metamorphosed sediments of late Algonkian time.
The Cambrian rocks lie upon this complex and the
period of intrusion is therefore between two
limits which are not very far apart. The lead
ratios of these minerals are far from being con-
stant, as the following examples show :

Yttrialite 1-15

Yttrialite 0-51

Mackintoshite 0-39

Uraninite 0*17

Fergusonite 1-04

Fergusonite 0-30

THE AGE OF THE EARTH

Boltwood found a satisafctory agreement in
four cases, the ratio being 0-17, but, as he pointed
out himself, most of the minerals from this locality
are unsuitable, because of incipient or advanced
alteration. The quart zose pegmatites in which
the minerals occur are riddled with alteration
products and secondary minerals, and the whole
series is altogether unfavourable to accurate age
determination. It is doubtful whether the apparent
agreement of the ratios quoted by Boltwood ought
to be accepted without further verification ; for
the present they cannot be regarded without
suspicion.

This example shows how the actual results
indicate the vicissitudes, varying from mineral to
mineral, which the lead and uranium contents
may have undergone. The method confirms or
denies the validity of its application in every case.
Judging from the relative solubilities of the con-
stituents in question, uranium is likely to be
abstracted from a mineral during the process of
weathering more readily than lead, and conse-
quently the age deduced from a weathered or
altered specimen should in general be too high.
A differential effect of this kind would account
for the high ratios given by the Llano Co. minerals.

Strict attention must be paid to the question of
origin, and secondary minerals avoided as care-
fully as altered primary minerals. Pitchblende is
often secondary, e.g. when it occurs in veins with
metalliferous sulphides. Other examples, of a
rather different type, are autunite and carnotite.
Secondary minerals are necessarily more recent than
the rocks in which they occur, and many of them

RADIOACTIVE MINERALS

date back to no very remote period. Autunite is
sometimes formed quite near the surface, within
a few inches in fact. Its antiquity cannot there-
fore be more than a few thousand years, and in
this time a detectable quantity of lead could not
be generated. The traces actually found were
probably in the original possession of the mineral.
In an analysis made by the writer, only 0-06 % of
lead was found in specimens of autunite from
Mozambique, where it occurs in bright green
flakes attached to the large biotite crystals of
pegmatitic dykes. In keeping with the age of the
pegmatites and the high proportion of uranium —
45 % — it should have contained a hundred times
as much had it been a primary mineral. The
paucity of lead in autunite has even been put
forward as an argument against the contention
that lead is the ultimate product of disintegration
of the uranium family. We now see how baseless
is this argument when the origin of the mineral
is remembered. It is not surprising that autunite
should contain so little lead ; on the con-
trary, it contains much more than the uranium
can account for in the time at its disposal.

From these considerations it will be obvious
that the only minerals to be chosen as material
from which to determine the lead-ratio are fresh,
stable primary rock-minerals. Having decided
this, there remains a third possibility which might
cast doubt upon the method. It can be objected
that lead may originate as a product of some
element other than uranium. Analytical results
show clearly that thorium cannot give rise to lead,
or a more proportionate relationship between

THE AGE OF THE EARTH

these two elements would have announced the
fact. There is also a possibility that certain of
the longer-lived members of the uranium family
may themselves be segregated in a mineral, in-
dependently of uranium. If so, they would gradu-
ally disintegrate, leaving no trace of themselves
other than the residual helium and lead. In a
magma containing 10 % uranium, the radium
would amount only to 0-0000034%. In actual
magmas, even of pegmatites, the quantity present
is always much less than this, and even if it be
allowed that such tiny quantities may saturate the
magmatic solution, the precipitation and concentra-
tion in any particular mineral would not be
sufficient to leave an appreciable residue of lead.

The application of the accumulation of helium
in minerals to the measurement of geological time,
was first suggested by Rutherford in 1905, when
he wrote : " I think that, when the constants
required for these calculations are more definitely
fixed, this method will probably give fairly trust-
worthy information as to the probable age of some
of the radioactive minerals of the earth's crust,
and indirectly as to the age of the rocks in which
they are found."

During the years 1908-10, Strutt examined a
great number of minerals, and determined the
helium ratio whenever practicable. His first set of
experiments dealt with phosphatic nodules and
phosphatised bones. These may sometimes contain
fifty times as much uranium as average rock
material, and they have a further advantage in that
they can be found in strata of nearly every age.
As they frequently consist of fossils characteristic

RADIOACTIVE MINERALS

of the formations in which they occur, their age is
well defined. However, the power of retaining
helium is both poor and variable in the case of
these phosphates, and the time relation is therefore
obscured. Such materials never retain more than
a small fraction of the helium which has been
generated within them.

More suitable in their power of retention are
certain iron ores, from which significant results
were obtained. The helium ratio, and therefore
the numerical age derived from it, showed a
marked dependency upon the geological age of
the mineral, as the following examples illustrate :

MINERAL.

GEOLOGICAL AGE.

MILLIONS OF
YEARS.

Sidcrite

Upper Oligocene

8'4

Haematite .

Eocene

30-8

Haematite .
Haematite .

Upper Carboniferous
Devonian

141-9
145-2

Strutt next investigated the more compact
minerals of igneous rocks, notably zircon and
sphene. Zircon can be obtained from rocks
belonging to several periods of igneous activity,
and being a durable and stable mineral it is pecu-
liarly fitted to retain the helium generated within
it. Even allowing that the helium found does not
represent the whole amount generated, it is
unlikely that the fraction lost will vary as con-
spicuously as in the case of phosphates. In so far
as that fraction depends on the structure of the

THE AGE OF THE EARTH

mineral it is probably more uniform for zircon than
for most other minerals. The helium ratio ought
therefore to stand in a close relation to the geo-
logical age of the specimen. That it does so is
clearly demonstrated by Strutt's results, which
are given below. The geological ages have been
taken from the most recent literature, and are
given in greater detail than those published in the
original paper. Where two periods are bracketed
together they are to be understood as referring
to the limits between which the age of the igneous
rock may fall.

LOCALITY.

GEOLOGICAL AGE.

MILLIONS
OF YEARS.

Mt. Somma, Vesuvius .

r Recent
\Pleistocene

O.I

Mayen, Eifel . . ,

Pleistocene

i-o

Campbell I., N.Z. . .

Pliocene

2'5

Expailly, Auvergne

Miocene

6-3

Brevig, Norway

Devonian

Cheyenne Canon,
Colorado

( Upper Cambrian
1 Archean

141

Green River,

| Carboniferous

N. Carolina

( Archean

UralMts

Pre-Devonian

209

Ceylon

Archean

286

Blue Ground, Kimberley

Archean

321

Sebastopol, Ontario

Archean

622

The results for sphene add but little to the above
table, for most of the rocks from which workable
quantities of sphene can be obtained are of pre-
Cambrian age. The most notable helium ratio
was from a specimen occurring in the Archean
156

RADIOACTIVE MINERALS

rocks of Ontario, and corresponded to an age of
715 million years. Summarising all the data
afforded by Strutt's work, we may graduate the
geological column with a time scale. It must be
clearly understood, however, that the ages as
expressed in years are, in the case of the helium
ratio, minimum values only. How much greater
the time represented by the geological periods
actually is will appear from the ages as deduced
from the lead ratio. A few of these are placed in
the table below for comparison.

TIME SCALE IN MILLIONS OF YEARS.

J.HE GEOLOGICAL 5YST1

:MS.

HELIUM RATIO.

LEAD RATIO.

Pleistocene .

Pliocene

2'5

Miocene .

6-3

—

Oligocene

8-4

—

Eocene

30-8

—

Cretaceous .

Jurassic

—

Triassic

—

. —

Permian

Carboniferous

146

34°

Devonian

*45

37°

Silurian .

)

Ordivician

} 43°

Cambrian

209

Algonkian

IOOO-I2OO

Archean

710

1400-1600

The application of the lead-ratio to the measure-
ment of the antiquity of minerals was first due to

THE AGE OF THE EARTH

Boltwood. He tested its reliability by an appeal
to the best analyses published up to 1907. Some
of these, with geological details, will now be
given.

In Glastonbury, and also in Portland, Connecti-
cut, primary uraninite is found in the felspar
quarries. The pegmatite in which the mineral
occurs is associated with a granite which intrudes
Lower Carboniferous strata. It is probably to be
referred to the close of the Carboniferous period,
and is certainly pre-Triassic. Five different speci-
mens gave lead - ratios in striking agreement,
corresponding to an age of 340 million years.

It should be observed in all the following tables
that in calculating the lead-ratios the time-average
of uranium has been used, and not the amount
actually present. The statement of analyses is,
of course, in percentage.

URANIUM.

LEAD.

RATIO.

2-9

0-041

3-0

0-042

2-8

0-039

3-0

0-041

2-9

0-040

Similar crystals of uraninite have been furnished
by the pegmatites of Branchville, Connecticut.
The intruded strata are of either Silurian or
Ordivician age, and the evidence suggests that the
period of intrusion is possibly coincident with that
of the earth movements which commenced at the

158

RADIOACTIVE MINERALS

close of the Ordivician. Here again the lead ratios
closely agree, the age being 430 million years :

URANIUM.

LEAD

RADIO.

4-0

0^052

4-0

0*051

4-0

0-052

3'5

0-051

In North Carolina uraninite occurs in coarse
pegmatites which are mined for their large flakes
of mica. In this instance a good agreement is
scarcely to be expected, as secondary products
abound, and the three specimens from Spruce Pine
which were examined by Hillebrand showed signs
of incipient alteration. The fourth specimen was
from South Carolina and • this again lacked the
freshness which is so essential. Zircon is also found
in the North Carolina pegmatites, and the writer
has examined two sets of specimens with the results
given below. The material in this case appeared
to be quite fresh.

The geological period is difficult to establish
with certainty. The relations of the Appalachian
rocks of Carolina are so obscure that the required
age may be anywhere from pre-Cambrian to
Carboniferous. Judging from the lead ratios,
one is tempted to suggest that the age is not far
from the Silurian, but until our time-scale is better
determined one must be chary in this converse
application of the method. It may confidently be
hoped that in this way the geologist will be greatly

159

THE AGE OF THE EARTH

helped in his attempt to unravel the history of
igneous activity in the earth's crust. But the time
is not yet, for the accumulation of facts and data
has only just commenced.

MINERAL.

URANIUM.

LEAD.

RATIO Pb/U.

Uraninite

3'9

0-049

Uraninite

4-2

0-052

Uraninite

3'3

0-047

Uraninite

3'3

0-045

Zircon

0-076

0-0036

0*046

Zircon

0*130

0-0055

0*041

Other series of minerals could be given from the
pre-Cambrian rocks of North America, Ceylon,
and Mozambique, but it is unnecessary to multiply
details further. In all cases the Archean seems
to date from 1200-1600 million years.

We now turn to the pre-Cambrian rocks of
Scandinavia and Finland, and for comparison
with those of N. America the following classifica-
tions and correlations may be given. The chief

Fennoscandia. North America.

^Algonkian

•Archean

Jotnian

Keweenawan

Jatulian . .

Upr- Huronian

Upr. Kalevian .

Mid. Huronian

Lr. Kalevian . .
Bottnian

Lr. Huronian

Ladogian
Katarchean . .

Laurentian
Keewatin

160

RADIOACTIVE MINERALS

unconformities are indicated by wavy lines. It
should be pointed out that the correlation of the
pre-Cambrian rocks over wide areas is, perhaps,
the most difficult task the geologist has to attempt,
and the scheme given opposite is based more on
analogy than direct evidence.

It is not yet possible to support this correlation
by a concordant system of chronology. Igneous
rocks occur at most of the horizons, but it is
extremely difficult to get samples of suitable
mineral species for analysis. However, there is no
doubt that in the future a definite time estimate
will be attached to each of the above periods.
When we are in full possession of this knowledge,
and only then, will a reliable correlation of these
rocks be possible.

The analyses collected by Boltwood include two
groups which are of minerals taken from the
pegmatites of Southern Norway, rocks famous for
the occurrence of rare minerals. The first group,
from the igneous complex of the Moss district, of
which the average age is 1000 million years, is as
follows :

MINERAL.

URANIUM.

LEAD.

RATIO.

Uraninite

8-4

0-12

Uraninite

7-8

O'll

Annerdodi

2-2

0'135

Uraninite

9'3

0-13

Uraninite

8-0

OT3

Uraninite

8-8

0-135

Uraninite

8-8

O'I2

Uraninite

9-0

O'll

Thorite

8-2

1-2

0-I3

161

THE AGE OF THE EARTH

The second group, from the complex of Arendal,
demands an age of 1200 million years. The Scan-

MINERAL.

URANIUM.

LEAD.

RATIO Pb/U.

Uraninite

9-8

0-14

Uraninite

IO-2

0-15

Uraninite

9*4

0-15

Thorite

i'5

0-16

Orangite

7'5

1-2

0-15

Xenotime

2-9

0-62

0-19

dinavian geologists believe that both groups of
rocks are younger than the quartzites and other
metamorphic sediments with which they are
always associated, and that the latter rocks are
of late Archean age. Sederholm, however, thinks
they may be equivalent to his Kalevian division.
The geological evidence, as far as it goes, does not
point to any difference in the ages of these two
sets of Norwegian minerals, but, on the other
hand, there is no positive evidence that the
ages are the same. An analogy made in
ignorance cannot be held to constitute a proof.
Field work in this case does not disqualify the
testimony of the radioactive minerals ; it rather
invites their co-operation in the perplexing task of
disentangling the intricate structural relations of
the rocks.

Amongst these rocks correlation is exceedingly
difficult, and even their relative ages are hidden
in obscurity. Hb'gbom holds that the massifs of
Moss and Arendal are contemporaneous with the
Ser-archean granites of Sweden. Similar hyperites

162

RADIOACTIVE MINERALS

and quartzites are found in nearly every locality
and the granites always appear to be younger
than these. After their intrusion, an enormous
thickness of rock was denuded away before the
Jatulian sediments were laid down. The physical
break here indicated is one of the greatest in the
history of the earth, and undoubtedly represents
an immense lapse of time. The granites and
pegmatites must therefore be considerably older
than the Jatulian rocks.

With a view to testing the constancy of the lead-
ratio in a series of minerals from a single igneous
complex, the author, in 1911, made a number
of experiments on carefully chosen material.
There occurs in the Christiania district of Nor-
way, a geologically depressed area of nearly
4000 square miles, which is separated by faults
from the surrounding pre-Cambrian rocks on every
side. Within this area there is a nearly complete
sequence of early Palaeozoic rocks, surmounted by
a few beds of red sandstone of Lower Devonian
age. Over these beds and intercalated with them
are lava flows ; and finally, penetrating the whole
mass, and representing a later phase of the same
period of igneous activity, are great intrusions of
plutonic rocks. Amongst the earliest of the intru-
sions is a series of thorite-bearing nepheline-
syenites. Brogger believes them to be of Middle
or Lower Devonian age, most probably the latter.
The minerals occurring in them are, in many in-
stances, notably radioactive, and thus they afford
an admirable series in which to investigate the
consanguinity of lead and uranium. A suite of
minerals was obtained from Brevig, and estima-

163

THE AGE OF THE EARTH

tions of these elements made, with the following
results :

MINERAL.

URANIUM.
GRS. PER ioo
GRS. MINERAL

LEAD.
GRS. PER IOO
GRS. MINERAL

Pb/U

Thorite

10-1040

0-4279

0-042

Orangite .

1-2437

0-0570

0-046

Orangite .

1-1825

0-0542

0-046

Thorite .

0-4072

0-0196

0-048

Homelite .

0-2442

O-OI2I

0-049

Zircon

0-1941

0-0085

0-044

Pyrochlore

0-1923

O-OI2O

O-O62

Pyrochlore

0-1855

O-OO93

0-050

Biotite

0-1602

O-OO69

0-043

Tritomite

0-0631

O-OO26

0-041

Freyalite .

0-0526

O-OO28

0-053

Mosandrite

0-0432

0-0024

0-056

Aegerine .

0-0253

0-0015

0-060

Astrophyllite
Catapleite

0-0140

0-0132

O-OOO7
0-OOO9

0-050
0-068

Nepheline

o-ooio

O-OOO4

0-400

Felspar

O'ooo6

O-OOO3

0-500

It will be noticed, that with a few exceptions,
the value of the ratio increases as the percentage
of uranium diminishes. This is probably due to
the relative importance of lead originally entangled
in the minerals at the period of their crystallisation.
Thus it would seem in the case of nepheline and
felspar that almost the whole of the lead found
was originally present, while that which has since
been generated is very small in comparison.
Minerals with so little uranium contain too much oc-
cluded lead to be reliable, and are, of course, value-
less in age-estimations. When sufficient uranium
is held by a mineral, the lead generated becomes
164

RADIOACTIVE MINERALS

increasingly important, until the original amount
is of negligible consequence. There is always the
possibility that in some of the richer minerals larger
quantities of lead were occluded than in felspar,
but the agreement among the ratios renders this
improbable. Rejecting all the results after that
of biotite because of the low percentage of uranium,
and omitting that of the first specimen of pyro-
chlore because the estimation could not be verified
owing to lack of material, the mean ratio is 0-046.
Replacing the uranium percentage by its time-
average value, the ratio becomes 0-045 and the
corresponding age 370 million years. It may be
thought somewhat arbitrary to select certain
results preferentially, but in view of the inter-
pretation placed upon them the choice is not
unfair, nor without justification.

Most of the available evidence drawn from radio-
active minerals has now been passed in review.
As yet it is a meagre record, but, nevertheless, a
record brimful of promise. Radioactive minerals,
for the geologist, are clocks wound up at the time
of their origin. After a few years' preliminary
work, we are now confident that the means of
reading these time-keepers is in our possession.
Not only can we read them, but if they have been
tampered with and are recording time incorrectly,
we can, in most cases, detect the error and so
safeguard ourselves against false conclusions.

165

CHAPTER XI
REVIEW OF THE EVIDENCE

The discrepancy between the geological and radioactive
methods of estimating time — Uniformity of the rate of
decay of uranium — Joly's criticism — Comparison of the
two time-scales — Doubtful assumptions made in the
geological arguments — Possibility of reconciliation no
longer hopeless.

OF the various methods which have been
devised to solve the problem of the earth's
age, only two, the geological and the radioactive,
have successfully withstood the force of destructive
criticism. The other arguments may be dismissed
without further discussion, as in every case their
cogency has been vitiated by the detection of a
fundamental error. From the mists of controversy
which for half a century have hung over the
subject, the two hour-glass methods alone emerge,
and the final issue must be fought out between
them. In the one the world itself is the hour-glass,
and the accumulating materials are salt, the
sedimentary rocks and calcium-carbonate. Three
concordant sets of results may be drawn from this
triple scheme of measurement, but it must not be
supposed that they are altogether independent.
Each set of data is intimately related to the others
and all stand or fall together. In the other case
the accumulating materials are helium and lead,
and the hour-glass is constituted by the minerals

166

REVIEW OF THE EVIDENCE

in which they collect. Provided that the field-
evidence is clear and convincing and that the
relative geological age of a mineral specimen can
be determined, the construction of an exact and
precise time-scale is a task which can be dealt with
successfully in the laboratory. The problem has
advanced from the qualitative to the quantitative
stage, and for the first time in historical geology
accurate measurement founded on delicate experi-
mental work has become possible.

It is a matter for regret that confidence in this
pioneer work has been shaken by the advocates
of the geological methods of attack. The surprises
which radioactivity had in store for us have not
always been received as hospitably as they deserved.
With the advent of radium geologists were put
under a great obligation, for the old controversy
was settled overwhelmingly in their favour. But the
pendulum has swung too far, and many geologists
feel it impossible to accept what they consider the
excessive periods of time which seem to be inferred.
That there exists a serious discrepancy obviously
points to a flaw in the underlying assumptions of
one or the other or both of the methods. Evidently
we are at the parting of the ways. The funda-
mental assumptions on which the arguments are
based cannot both be right. One of them must be
rejected. Which is it to be ? Let us consider each
in turn, and discuss the consequences of the two
possible forms of reconciliation.

The only assumption which can reasonably be
called into question is that of uniformity, and it
is involved equally in both calculations. It is here,
at the root of the problem, that the discrepancy

167

THE AGE OF THE EARTH

really lies. If we favour the uniformity of geological
processes — a well-worn doctrine which has done
good service — then we must reject uniformity of
radioactive disintegration. Joly has drawn
attention to the latter possibility. He asks: Is
it assured that the parent substance, uranium, has
always in the past disintegrated at the rate
determined by its present average life-period ? As
far as we know, the rate of decay for substances of
rapid transformation is constant, and independent
of temperature and pressure changes. On the
grounds that a large number of radioactive bodies
decay at a constant rate, it is believed that this con-
stancy is a definite attribute of all the radioactive
elements. In the case of uranium this assumption
cannot be proved for periods commensurate with
its half-life period. On analogy with the behaviour
of the shorter lived elements, it is probable that had
we lived in Cambrian times and experimented with
Archean uranium-bearing minerals just as has
been done during the last decade, the half-life
period would then have been exactly the same as
we now find it — about 5400 million years. In the
case of radium emanation there can be no doubt
that experiments in Cambrian times would have
given results concordant with ours. It would be
as unphilosophic to doubt this as to believe that
the laws of physics and chemistry vary with time.
The difference between uranium and its daughter
elements, the difference which suggests to Joly
a possible distinction, is simply one of origin. We
are in complete ignorance of the genesis of uranium.
It is not impossible that, owing its origin to some
process other than atomic transformation, the

168

REVIEW OF THE EVIDENCE

particular distribution of intrinsic energy among
its atoms may not be such as to maintain a
constant rate of decay. At the moment of its birth
every radioactive atom has a definite expectancy
of life, and when a sufficiently large number of
atoms is under observation a definite fraction dis-
integrates every second. There is this difference
in the case of parent elements. As they become
aged with reference to the time of their origin,
they are not reinforced by the addition of fresh,
newly-born atoms, as are the other members of
each series. Joly's supposition seems to be that
in the absence of this reinforcement the uranium
in its early stages may possibly disintegrate more
rapidly than it does now. However, it is not found
that the younger atoms of the short-lived elements
are, on an average, more prone to rapid decay than
are their older companions. Whether an element
is in equilibrium with the higher members of its
family, or whether it is separated from them, its
transformation proceeds with unaffected regu-
larity.

It is very improbable that reconciliation will be
found in the supposition of a progressive retarda-
tion of the rate of decay of uranium. There are
three possibilities. Uranium may have disinte-
grated in the past exactly as it now does ; or it
may have decayed more slowly or more rapidly.
The latter two alternatives do not favourably
commend themselves. There is no evidence which
can be cited in their support. On the other hand,
the hypothesis of constant change is deduced from
a well-established series of experimental facts, and
is remarkably in accordance with the general

169

THE AGE OF THE EARTH

phenomena of radioactivity. Uranium, in other
respects, does not present any anomaly, and with
regard to the mechanism of its decay physicists
are not likely to regard it as an exceptional case
without very definite reasons for doing so. The
discordance between the time-estimates drawn
from the rates of geological and radioactive
changes cannot be held to constitute a sufficient
reason for rejecting current opinions unless it
is conclusively demonstrated that the geological
estimates are beyond question. In the future
the case for uranium may be established
more securely, when the dynamics of atomic
disintegration, and the conditions upon which
the distribution of unstable atoms depends, be-
comes more intimately understood. At present
there is only one means of testing the constancy of
uranium decay, but unfortunately it affords only
negative evidence. The range of a-particles from
a radioactive element is connected in some way
with its rate of decay. If then, uranium in the
past disintegrated more rapidly, the radius of its
particular pleochroic halo ought to record the
difference. It is improbable that any variation —
assuming for the moment that there were a
variation — could be detected even in the most
favourable cases.

We now turn with a double interest to the
geological estimates. If it can be shown that they
ought to be largely increased, as Chamberlin
and a few other geologists believe, then not
only is a reconciliation at once made possible,
but, in turn, the constancy of uranium decay
is placed beyond doubt. Little need be added
170

REVIEW OF THE EVIDENCE

to the discussion in Chapter VI. It was there
indicated that one factor previously over-
looked— the average height of the continents in
geological time — very largely controls the rate of
denudation and therefore of sedimentation. Let us
make an attempt to discover how past rates must
be related to those of the present to make possible
a complete reconciliation. In Fig. 17 the dis-
crepancy is illustrated graphically by comparing
the respective time-scales from the close of the
Archean (gneiss and granite phase) to the present
day. All the sediments of which relics have
remained to us are in this way taken into con-
sideration. Lying buried in the Archean, the base
of the record is obscured beyond recognition by the
prevalence of metamorphic and plutonic igneous
rocks. The extent to which the earliest sediments
have been lost in the evolution of the earth's crust,
and the part they have played in the genesis of
granites and gneisses are questions which betray
our ignorance and offer food merely for wild
speculations. These possibilities, however, do
not touch the immediate point at issue, for in their
time relations they lie outside the limits to which
this discussion is restricted.

The curve A is plotted strictly against the
maximum observed thickness of sediments, and
corresponding to it is the sedimentation line A' to
which is granted 300 million years. According to
these two graphs the greatest error lies beyond the
Cambrian. The average rate of denudation and of
sediment accumulation must now be nine times
that of the pre-Cambrian periods, but if post-
Cambrian is compared with present the ratio is

171

§ B

REVIEW OF THE EVIDENCE

reduced to two and a half. In the B series, the
lead ratios are plotted in a straight line and the
stratigraphical column is extended in accordance
with the palaeontological evidence that pre-
Cambrian time is at least as long as that which has
elapsed since the beginning of the Cambrian. On
this basis present rates are four times the average
for post-Archean time.

Assuming that the true time-scale lies some-
where between the extremes of A and B, we are led
to two conclusions which, if accepted, greatly
lessen the severity of the discordance between A
and A' and B and B'. From A and A' it appears
that pre-Cambrian denudation took place much
more slowly than has since been typical. This pro-
position is in complete accordance with the view
that the pre-Cambrian continents, when viewed in
the light of the reconstructed geographies of the
later periods, were of limited area and restricted
elevation. From B and B' the broad time con-
ception of palaeontology gains further support,
and the existence of great gaps in the
pre-Cambrian succession is suggested. It is well
known that the most important unconformities of
the whole geological record are to be found in the
imperfect succession of the earliest formations. It
is impossible to do more than guess at the duration
of time periods which are without their sedimen-
tary equivalents. On the most extreme assump-
tion, the Algonkian sediments should be represented
not by 82,000 feet, but by more than 300,000 feet.
Unwilling though we may be to consider the
record imperfect to this incredible degree, it is of
importance to point out that had such an immense

173

THE AGE OF THE EARTH

thickness existed in successive periods of time, the
sodium content of the ocean would not necessarily
be in any way different from what it is. The same
amount of primary rock material may have been
broken up, but instead of a reasserting of the
materials to form three successive sets of sediments
(on a rough average) we would be obliged to postu-
late six or more repetitions of the sorting process.
The question need not be pursued farther. The
suggestion here put forward is, that in the limita-
tions of pre-Cambrian geography and in the
imperfection of the sedimentary relics of those
remote times, the discrepancy which is peculiar to
the pre-Cambrian finds an adequate explanation.
If this be allowed, all that remains is to decide
whether it is inconsistent with geological principles
to assert that the modern hour-glass is running at
two-and-a-half to four times its average rate.
The decision depends largely on the broad point
of view from which geological interpretatior
proceeds. From the standpoint of Catastrophisn.
little progress was made. Uniformity proved a
great advance, but in detail it is apt to lead us
astray if applied too dogmatically. Modern
interpretation is based on the more philosophic
conception of Evolution, and in place of the earlier
idea which was insisted upon by the older physicists
— that changes have been such as would accom-
pany a gradual running down of the earth's
internal kinetic energy — the form of development
now favoured is that of cycles of phenomena,
recurring in their broad features again and again
and not necessarily hampered in their activity by
any progressive diminution in the store of available

174

REVIEW OF THE EVIDENCE

energy. Igneous action, deposition of sediments,
marine transgression and recession, are all rhymic
phenomena and the factor common to each one,
whether as cause or effect, is earth-movement.

The conditions of the present day cannot then
be accepted as representing average conditions,
unless it were by a happy accident. Amidst all
the details of earthquakes and volcanic eruptions
some great cycle is now running its course, and only
in relation to the particular phase of the cycle under
which we happen to pursue our investigations will
our conclusions be strictly tenable. Although we
:annot hope to judge the exact place which the
^resent takes in the larger scheme of terrestrial
activity, yet in comparison with the past, the
present epoch would seem to approach just those
extremes most favourable to a high rate of denuda-
;ion, and to a rapid accumulation of sediments.
Marine recession, brought about by deepening
f the ocean basins, and raising of the land areas
^ave together brought about continental expansion
aid elevation. The vulcanism of the present day,
whether regarded as a closing phase of a period of
igneous action, or as the initiation of a new cycle,
probably affords an example of more than average
intensity and violence. The weathering capacity of
rain must be enhanced in proportion to its content of
dissolved acid gases, and this in turn is conditioned
by the prevalence of vulcanism. Still another
factor leading to higher rates, though of a different
category, is due to recent glaciation. Over wide
areas easily eroded deposits are exposed, the areas
being generally those which would resist denunda-
tion most successfully. In Fennoscandia, for

THE AGE OF THE EARTH

example, four-fifths of the pre-Cambrian shield is
buried beneath a thin covering of moraine.

It is not suggested that present rates have never
before been reached, but only that they are
characteristic of the more intense phases of
denudation rather than of average conditions. If
this be granted, reconciliation of the rival time-
estimates is no longer hopeless. There can be no
doubt that agreement will never be brought about
by the more convincing testimony of experimental
demonstration. It must be almost entirely a
question of interpretation. An attempt has been
made to show that in the geological evidence there
is nothing impossibly at variance with the dictates
of the radioactive minerals. With the acceptance
of a reliable time-scale, geology will have gained
an invaluable key to further discovery. In every
branch of the science its mission will be to unify
and correlate, and with its help a fresh light will
be thrown on the more fascinating problems of
the Earth and its Past.

176

APPENDIX A
(a) Kinetic Energy of ^-particles

i gram of radium in equilibrium with emanation,
Ra. A B and C generates heat at the rate of 132
calories per hour (85% due to a-particles).

e =charge on a-particle =9-3 x io~10 E.S. units.

= 3-1 x io-20 E.M. ,,
m =mass of a-particle 1
v velocity of a-particle } see Table below'
N= number of a-particles liberated from i gram of

radium =3-4 x io10 per second.
Energy E transformed per second is given by —

_Ne 2mv2
~ 2 e

Element.

Cms. per sec.

mva/e

E.M. Units.

Radium

1-56 x io9

4-78 xio14

Ra. emanation
Ra A

1-70 x io9

I '11 X IO9

5-65 x io14

6'12 X IO14

Ra C . ...

J. f 1 /\ i-\.

2-06 x io9

8-37 x io14

Substituting these values, we have —

Ne=3'4 xio10X3-i x io-20 = io-5 x io~10E.M. units.

S™2 - ioi4(4-78 +5-65 +6-12 +8-37)

= 24-9 x io 14 E.M. units ;
whence £ = 13-1 xio5 ergs per second
= 4-73 x i o9 ergs per hour.

THE AGE OF THE EARTH

Now 4-19x10' ergs = i gram-calorie.
.-. E = 113 calories per hour.

(b) Production of helium from Uranium and Thorium in
equilibrium with all their disintegration products.

Uranium —

N = number of helium atoms liberated from i gram
of radium alone =3-4 x io10 per second. (Rutherford
and Geiger, 1908).

The equilibrium ratio of radium to uranium is
3'4Xio-7. Hence for each gram of uranium in
equilibrium the number of atoms produced amounts

3-4 x io10X3*4 xio-7x8 per second
=29-1 x lo11 per year.

Now the number of helium molecules, and therefore
of atoms, in i cc. of the gas at N.P.T. is 2-72 x io19.

The annual production of helium must conse-
quently be 29-1 x io11 / 272 x io19 ccs., i.e. 107 x
io-8 ccs., or 1-88 xio-11 grs. per gram of uranium.
An experimental determination gave 10-6 x io-8 ccs.
(Strutt, 1910).

Thorium —

The ionising power, or the energy of the a-par-
ticles from i gram of thorium, is 0-325 of that from
i gram of uranium, each element being in complete
equilibrium.

Average range of a-particles from thorium and its
products =5 -4 cms.

Average range of a-particles from uranium and its
products =4-3 cms.

The average thorium a-particle is therefore 1-25
times as energetic as the average uranium a-particle.

Hence the actual production of a-particles or
helium atoms from thorium is only o-325/i'25 ^=0*26
of that of uranium. Experimental determinations
0-23 (Strutt, 1910), 0-27 (Rutherford and Geiger, 1910).

178

APPENDIX A

(1) The number of a-particles emitted from i gram
of radium per second (n=3'4Xio10) is equal to the
number of atoms disintegrating per second;

If N is the number of atoms in i gr. radium, then X,
the fraction which transforms per second, is given

by— X=n/N.

The number of atoms in i gr. hydrogen is
6-24 x io23,

and as the atomic weight of radium is 226 times that
of hydrogen, N = 2-y6 x io21.

.-. X = 1-25 x lo'11 gr. per sec.
= 3-94 x io" 4 gr. per year.
Half- Life period =o -693 15/X
= 1760 years

(2) i gr. of radium is in radioactive equilibrium
with 0-58 cubic millimetre, or 5-yxio-6 grs. of
emanation (atomic wt. =222).

If Xj = 2-085 x io~6 is "the fraction of emanation
transforming per second, we have —

X=5-7xio-6XX1

= 1-19x10 " gr. per sec. ;
whence —
Half- Life period = 1850 years.

The earlier values given for the half-life period
were 1760 and 2000 years, but the lower figure seems
most accurate, with 1850 years as a probable value.

The half-life of uranium would then be —

1850/3-4 x io~7=540o million years.

(d) Time Average of Uranium

Ut= Quantity of uranium remaining after a time t.
U0 = Quantity of uranium originally present (t =o).
179

THE AGE OF THE EARTH

Um = Time-average of uranium during time
X= Disintegration constant of uranium.
Pbt=Lead accumulated during time t.
Het= Helium „ „ time t.

0-9

lo,
o

3> "Time

Graph I represents the rate of decay of uranium-
according to the exponential law —

Ut =U0e-Xt,

There is one rate of decay which, if it remained
constant throughout the time t, would have a total
effect equivalent to that produced by the actual

180

APPENDIX A

slowly decreasing rate of decay. This average rate
is represented by some point on the curve, and the
corresponding quantity of uranium, Um, is the time-
average. Equating the amount of uranium trans-
formed in each case, we have —

U0-Ut=XUmt
whence Um = (U0 -Ut)/Xt (a).

)

1 10
108
1-06

too

Saa.

>h I

> aoo <*oo eoo goo 1000 12.00 1400 1600 iffoo zo
>• Time . u.rut = one million years

For periods less than 2000 million years, the time-
average is nearly equal to the arithmetic mean of
U0 and Ut. Um =(Uo+Ut)/2 .(6).

The value of Um for 2000 million years is,

according to equation (a), equal to 0-874 U0
and ,, ,, ,, (b) ,, ,, 0-879 U0.

The ages of minerals rarely exceed 1500 million
181

THE AGE OF THE EARTH

years, and therefore the error involved by using
equation (a) in preference to (b) is quite negligible.

In Graph II the ratio Um/Ut) i.e. the factor by
which the present uranium content of a mineral must
be multiplied in order to obtain the true time-
average, is plotted against time.

An approximation to the age, t, of a mineral is
afforded by the ratio Pbt/Ut< From the graph the
factor corresponding to this time can be obtained,
and thence the time-average. This in turn can be
utilised to give the more correct age represented by
Pbt/Um.

A more straightforward method of correction is as
follows :

U0 can be determined from known quantities ac-
cording to the following equation :

U0=Ut+Pbt+Het

=Ut + i-i5Pbt;

whence, Um = Ut + o'575 Pbt.

The age of the mineral is then given by the ratio
Pbt/Um, or directly from Graph II.

(e) Analyses made by the Author of Radium in Igneous
Rocks (cited on p. 130)

A cid Rocks — Ra. per gram of rock

Granite, Mozambique ........ 5-84 x io~12 grs.

„ .......... 2-61

N. Nigeria ....... 3-09

Rhodesia ......... 2-43

Transvaal ........ 2-12

South Africa ...... 1-81

...... 2'73

182

APPENDIX A

Basic Rocks —

Basalt, Mozambique 0-94 x io~ 12 grs.

Dolerite ,, 0-85 ,,

Gabbro ,, 1-07 ,,

Norite ,, 0-54 „

Ultrabasic Rocks —

Composite analysis of 10 specimens from
Scotland, New Zealand, Africa,
and Canada 0-51 x io~ la grs.

183

APPENDIX B

BIBLIOGRAPHY

CHAPTER I

Thomson (Kelvin). Rep. Brit. Ass., p. 1819. 1855.

Proc. Glasgow Phil. Soc., Vol. IV, p. 272. 1860.

Phil. Mag., February, 1862, p. 158. 1861.

Popular Lectures and Addresses [P.L.A.], Vol. I,

p. 349. 1862.

Natural Philosophy, Appendix D.

P.L.A., Vol. II, p. 6. 1865.

-P.L.A., Vol. II, p. 10. 1868.
Huxley. Presidential Address, Quart. Journ. Geol.

Soc. 1869.
Thomson. P.L.A., Vol. II, p. 73. 1869.

P.L.A., Vol. II, p. 238. 1876.

P.L.A., Vol. I, p. 369. 1887.

King. Am. Journ. Science, p. i. 1893.
Correspondence in Nature. Perry, Kelvin, Tait.

January 3rd, March 7th, April i8th. 1895.
Poulton. Rep. Brit. Ass., p. 808. 1896.
Kelvin. Phil. Mag., January, 1899, p. 66. 1897.
Correspondence in Nature. 1903.

Wilson. July gth.

Darwin. September 24th.

Joly. October ist.

Correspondence in Nature. Kelvin, Strutt, Lodge,
etc. September 2oth. 1906.

For other references see below.

CHAPTER II

Chamberlin and Salisbury. Geology, Vol. II, Chaps.
I and II. 1909.

184

APPENDIX B

T. C. Chamberlin and other Writers. The Tidal
Problem. Carnegie Inst. of Washington.
Pub. No. 107. 1909.

Darwin. The Tides. 1911.

Poincare. Hypotheses cosmogoniques. Paris. 1911.

CHAPTER III

Croll. Phil. Mag. May, 1868.

Climate and Time. 1875.

De Geer. Geol. Foren i Stockholm Fohr Band 32. 1910.
Gilbert. Journ. of Geol., Vol. Ill, p. 121. 1895.
Lowell. Evolution of Worlds, p. 197. 1910.
Sederholm. Bull. Comm. Geol. de Finlande, No. 30,

pp. 7-15. 1911.
Sollas. Ancient Hunters. 1911.

CHAPTER IV

Babb. Science, Vol. XXI, p. 343. 1893.
Clarke. Data of Gco. Chemistry. Geol. Sur., U.S.A.
Bull. 491.

- Smith. Misc. Collect., Vol. LVI, No. 5. 1910.
Dittmar. Challenger Report, Vol. I, p. 203. 1884.
Dole and Stabler. Water Supply Papers. Geol.

Sur., U.S.A., Nos. 234 and 236. 1910.
Geikie. Trans. Geol. Soc., Glasgow, p. 153. 1868.
Humphreys and Abbott. Physics and Hydraulics oj

Mississippi River, p. 148. 1876.
Merrill. Rocks, Rock weathering and Soils. 1897.
Murray. Scot. Geog. Mag., Vol. Ill, p. 65. 1887.

- Scot. Geog. Mag., Vol. IV, p. 38. 1888.
Reade. Chemical Denudation in Relation to Geological

Time. 1901.

Russell. River Development. 1898.
Schwarz. Causal Geology. 1910.
Tylor. Phil. Mag., April, p. 258. 1853.

185

THE AGE OF THE EARTH

CHAPTER V

Ackroyd. Chem. News, p. 265. 1901.

Geol. Mag., p. 445 and p. 558. 1901.

Becker. Science, Vol. XXXI, p. 459. 1910.

- Smith. Misc. Collect., Vol. LVI, No. 6. 1910.
Clarke. Smith. Misc. Collect., Vol. LVI, No. 5. 1910.
Proc. Am. Phil. Soc., p. 214. 1912.

Fisher. Geol. Mag., p. 124 and p. 132. 1900.
Halley. Phil. Trans., Vol XXIX, p. 296. ^715.
Hanamann. Landesdurchforschung Bohmens. Archiv.

Natur., Vol. IX. Vol. X. 1894. 1898.

Cited in Clarke's Data, 2nd ed., p. 92.

Joly. Trans. Roy. Soc., Dublin, Vol. VII, p. 23. 1899.

Rep. Brit. Ass., p. 369. 1900.

Geol. Mag., p. 344 and p. 504. 1901.

Radioactivity and Geology, p. 236. 1909.

Phil. Mag., p. 357. September, 1911.

Karsten. Inaug. Diss., Kiel. 1894.

Rudzki. Bull. Acad. Sci., Cracovie. February, 1901.

Sollas. Presidential Address, Quar. Journ. Geol.

Soc., Vol. LXV. 1909.

CHAPTER VI

Blake. Geol. Mag., p. 72. 1903.
Croll. Stellar Evolution, p. 48. 1889.
Geikie. Rep. Brit. Ass. 1892.
— Nature. August 4th, 1892.
— Rep. Brit. Ass., p. 727. 1899.
Haughton. Manual of Geology, p. 101. 1871.

- Nature, p. 266. 1878.
Holmes. Nature. July 6th, 1911.
Joly. Radioactivity and Geology. 1909.

de Lapparent. Bull. Soc. Geol. de France, Vol. XVIII,

p. 351. 1890.
Geologic, p. 225, p. 261, and p. 1958. 1906.

1 86

APPENDIX B

Lyell. Principles. Tenth Edn., Vol. I, p. 301. 1867.

Me Gee. Science, Vol. XXI, p. 309. 1893.

Phillips. Life on the Earth, p. 119. 1860.

Reade. Geol. Mag., p. 99. 1893.

Sederholm. Naturen, Helsingfors, Nos. 33 and 34.

1897-
Sollas. Nature. April 4th, 1895.

Rep. Brit. Ass., p. 711. 1900.

The Age of the Earth and other Geological Studies.

1905.
Presidential Address, Quay. Journ. Geol. Soc.,

Vol. LXV. 1909.
Upham. Am. Journ. Science, Vol. XLV, pk 217.

1893-

Walcott. Journ. Geol., Vol. I, p. 675. 1893.
Watts. Presidential Address, Quar. Journ. Geol. Soc.,

Vol. LXVII. 1911.
Wallace. Island Life, p. 222. 1892.
Winchell. World Life, Chicago, p. 378. 1883.

CHAPTER VII

Mme. Curie. Traite de Radioactivite. 1910.
Ramsay and Soddy. Proc. Roy. Soc. A ., Vol. LXXII,

p. 204. 1903.
Russell. Proc. Roy. Soc. A., Vol. LXXXVI, p. 240.

1912.

Rutherford. Radioactive Substances. 1912.
Rutherford and Soddy. Phil. Mag., Vol. IV, p. 582.

1902.
Rutherford and Geiger. Proc. Roy. Soc. A., Vol.

LXXXI, p. 151. 1908.
Rutherford and Royds. Phil. Mag., Vol. XVII,

p. 281. 1909.
Whytlaw-Gray and Ramsay. Proc. Roy. Soc. A.,

Vol. LXXXVI, p. 270. 1912.

187

THE AGE OF THE EARTH

CHAPTER VIII

Abbott. The Sun. 1910.

Arrhenius. The Life of the Universe, Vol. II, p. 193.
Helmholtz. Phil. Mag., p. 516. 1856.
Rutherford and Soddy. Phil. Mag. May, 1903.

CHAPTER IX

Becker. Bull. Geol. Soc. Am., p. 113. 1908.

Smith. Inst. Misc. Collect., Vol. LVI, No. 6.

Buchner. Proc. Konink Akad van Wetensch te Amster-
dam. October, 1910, February, 1911, and
April, 1912.

Chamberlin. Journ. Geol., p. 674. 1911.

Eve and Mclntosh. Phil. Mag. August, 1907.

Trans. Roy. Soc. Canada, p. 69. 1910.

Farr and Florance. Phil. Mag. November, 1909.

Fletcher. Phil. Mag. July, 1910.

Phil. Mag. January, 1911.

Phil. Mag. June, 1911.

— Phil. Mag. February. 1912.
Joly. Rep. Brit. Ass., p. 677. 1908.
Radioactivity and Geology, 1909.

— Phil. Mag. October, 1909.

Phil. Mag. July and August, 1910.

Phil. Mag. July, 1911.

— Cong. Internal, de Rad. and d'Elec., p. 370. 1911.

— Phil. Mag. February, 1912.
Phil. Mag. October, 1912.

King. Am. Journ. Science, p. i. 1893.

Strutt. Proc. Roy. Soc. A., Vol. LXXIX, p. 472.

1906.

- Proc. Roy. Soc. A., Vol. LXXXIV, p. 377. 1910.
Suzuki. Proc. Math.-Phys. Soc., Tokyo, p. 204. 1912.

1 88

APPENDIX B

CHAPTER X

Becker. Bull. Geol. Soc., Am., Vol. XIX, p. 113. 1908.
Boltwood. Am. Journ. Science, p. 260. 1905.

- Am. Journ. Science, Vol. XXIII, p. 77. 1907.
Boltwood and Rutherford. Phil. Mag., Vol. XXII,

p. 586. 1911.
Dewar. Proc. Roy. Soc. A., Vol. LXXXI, p. 280.

1908.

Proc. Roy. Soc. A., Vol. LXXXIII, p. 404, 1910.

Holmes. Proc. Roy. Soc. A., Vol. LXXXV, p. 248,

1911.
Joly. Radioactivity and Geology, p. 211. 1909.

Phil. Mag. September, 1911.

Krenigsberger. Geol. Rundschau, Vol. I, p. 245. 1910.
Rutherford. Radioactivity, p. 486. 1905.
Strutt. The Accumulation of Helium.

Proc. Roy. Soc. A., Vol. LXXXI, p. 272. 1908,,

Vol. LXXXII, p. 166. 1909.

Vol. LXXXIII, p. 96. 1910.

Vol. LXXXIII, p. 298. 1910.

-Vol. LXXXIV, p. 195. 1910.
Vol. LXXXIV, p. 380. 1910.

189

THE RADIOACTIVE

ELEMENTS

URANIUM FAMILY

ACTINIUM FAMILY

THORIUM FJ

[URANIUM, /^\

Ti-IOR!'JM

SI *-»««>• YEARS {) .

I3XIO* VEAHS

I URANIUM 2 /"~\

( I0» YEARS (f) L/*°

UR.Y /^"^V-

ACTINIUM

r~\

WESO- THORIUM 1

(?)

Y-

55 YEARS

URANIUM X /^~*V'

MCSO-THORiUM 2

24-6 DAYS V_y*r

6-2 HOUr:3

tONIUM / \

RADIO-ACTINiUM

RADIO-THORIUM

200,000 YEARS tfA./* "

13 5 DAYS

V. -A^a 2 YC/J.S

L8SO YEARS vJ^J

ACTINIUM *
IO-2 DAYS

THCPIL'M X

RA. EMANATION ^""^Wa

AC.tMANATiCN

/^^v

TH.EMANATIOM

3-85 DAYS V,^'

3-8 SECS.

I/**

54 gees.

RADIUM A ^~\».a
S MIHS V^X*

ACTINIUM A
0 002 SECS.

THOKIUM A
014 SECS

RADIUM B /^~V^

ACTINIUM 8

X \

THORIUM B

26-« WINS v>^r

38 M'NS

^^"

IC-G HOURS

RADIUM C.

ACTINIUM C

x-x

THORIUM Cl

105 MINS X_V^*r

2-1 MiKS

V-X *

•TR-O^I

TH.C, fV

(?) \^r

RADIUM o. RADIO-LEAD /^~~\ -

16-5 >£AR3 V/

ACTINIUM 0
4-71 MINS.

9::

THORIUM D
3-1 MINS.

RADIUM E f'~^\'^

(?)

5 DAYS \ Aiy-

RADIUM F, POLONIUM ^\,a

136 DAYS V^/

LEAD /^~\

STABLE I J

INDEX

a-particles, deflection of, 94

— nature of, 95

— penetration of, 94, 107, 108

— heating effect of, 96, 97

— kinetic energy of, 177

— atoms of helium, 96
Actinium and uranium, 100
Adams, 25

Age of earth, 86, 89, 125, 157

— minerals, 154-64

— moon, 15, 27

— oceans, 68, 89

— sediments, 86, 89

— sun, 112, 113, 120
Arrhenius, energy of sun, 117
Atmosphere, origin of, 25, 30

— as thermal blanket, 36
Atomic theory, 40
Atoms in I cc. helium, 140

— in i gr. hydrogen, 179

— in i gr. radium, 179

— disintegration of, 08, 102

— constitution of, 102
Autunite, age of, 100, 153

— radium-ratio in, 100

— lead-ratio in, 153

/3-particles, deflection of, 94

— nature of, 95

— from potassium, 103
Barus, diabase fusion-point,

125, 126

Barnes and Rutherford, heat
output of radium, 101

Basic rocks, radium in, 130

— distribution of, 128, 134
Becker, age of oceans, 62, 63,

— age of earth, 127
Becquerel, discovery of radio-
activity, 92

Becquerel rays, 93

Biology and the earth's age,
20, 21

Boltwood, constancy of radium-
ratio, 99

— origin of actinium, 100

— test of lead-ratio, 158
Boltwood and Rutherford, pro-
duction of helium, 139

Bragg, ionisation, 107
Buckner, radium in rocks, 130
Buffon, 23

Calcium carbonate, accumula-
tion of, 88, 89
Catastrophism, 4, 8
Chaldean chronology, 2
Chamberlin, planetismal hy-
pothesis, 28-30

— tidal theory, 27

— rate of denudation, 79, 80,
81

Challenger expedition, 85
Chlorine in rain, 66

— in rivers, 53, 70

— in sea, 53

— in rocks, 70

191

INDEX

Chronology, Chaldean, 2

— Persian, 3

— Hebrew, 3

— Indian, 3

— Byzantine, 4

Clarke, sodium in rocks, 67,
70, 71

— chemical denudation, 51-3

— volume of oceans, 63

— age of oceans, 63, 66
Cosmical energy, 120
Croll, glacial theory, 35-9

— marine erosion, 58

— age of earth, 86

Crust of earth, thickness of,
128, 134

— evolution of, 30, 134
Crystallisation of rock magmas,

145-8

Curie, Mme., radioactivity of
thorium, 92

— discovery of radium, 93
Curie and Laborde, heating

effect of radium, 96, 101

C. f>

uvier, o

Cycles of denudation and de-
position, 85, 174, 175
Cyclic sedimentation, 44

— circulation of salt, 66-70

Dana, volume of limestone, 88
Darwin, geological time, II
Darwin, Sir G., moon's his-
tory, 15, 19

— age of moon, 27
Decay, radioactive, 98

— physical independence of,
1 02

— possible variation with
time, 168

Deep-sea deposits, 59

Deluge, 4, 8

Denudation, initiation of,25,3 1

Denudation, chemical, 50, 54

— mechanical, 54

— marine, 58, 66

— present rates of, 78, 80
Deposition, 47, 81-5
Dewar, production of helium,

140
Diabase, fusion - point, 125,

126, 128

Dittmar, salinity of oceans, 64
Dole and Stabler, denudation

of N. America, 50, 55
Duane, heating effect of

radium, 101

Earth, origin of, 22 et seq.

— early history of, 25, 29, 73

— heat of, 12-16, 30, 122

— interior of, 134

— distribution of radium,

i3J-5

Earthquakes, 134
Electroscope, 93, 104
Emanation of radium, 97, 104
End products, 100, 140
Energy, conservation of, n,

in
Eve and Mclntosh, radium in

rocks, 130

Farr and Florance, radium in

rocks, 130

Fletcher, radium in rocks, 130
Fossils, early ideas, 5

7-rays, nature of, 94
Geer, De, glacial clays, 41-4
Geiger and Rutherford, count-
ing of a-particles, 138
Geikie, age of earth, 86

— marine erosion, 58

— on geological time, /8
Glacial periods, 33

192

INDEX

Glacial periods, chronology

of, 36 et seq.

Glacial clays in Sweden, 41
Glaciation, indications of, 33,

— cause of, 35

Gilbert, Cretaceous sediments
of Colorado, 44

Haloes, pleochroic, 107-9
Halley, salinity of oceans, 61
Heat, emission of, by radium,

96, 97, 101
by uranium, 116, 131,

132
by thorium, 116, 132

— escaping from earth, 122-4,

132

— of the sun, 1 10 et seq.
Helium, discovery of, 95, 96

— from radio-elements, 96

— rate of evolution of, 138

— leakage of, 149

— as end product, 149

— accumulation of, 154

in phosphates, 1 54

in iron-ores, 155

in zircons, 156

in sphenes, 156

Helium-ratio, as age-index,

143

Helmholtz, 12, 112
Hutton, 7, 22
Huxley, reply to Kelvin, 14,

Igneous rocks, exposures of,
71, 72, 73

— sodium in, 67

— radium in, 131

— thorium in, 131
lonisation, 93, 104
Ionium, 99

Iron meteorites, 134
Iron ores, helium in, 155

Joly, estimation of radium, 105
thorium, 107

— radium in rocks, 131

— thorium in rocks, 131

— age of the earth, 86

— age of the oceans, 17, 20,
62

— on radioactive decay, 168

Kant, 23, 26, 1 10

Kelvin, duration of earth's

heat, 12-17, I24» I25
sun's heat, 12-14, m

— tidal retardation, 13, 26,
27

Lamarck, 8
Land, area of, 52

— degradation of, 57
Laplace, 23, 28, 35
Laplacian hypothesis, 23-25

— difficulties of, 28
Lapparent, de, 86

Lead in igneous rocks, 147

— genetic connection with
uranium, 100, 140, 141

— in uranium minerals, 158-
164

— rate of production of, 142
Lead-ratio as age-index, 143
Lowell, Polar caps of Mars, 39
Lyell, Principles, 9

Mars, Polar caps of, 39
Mass of electron, 95
— of hydrogen atom, 179
Mayer, sun's heat, 12, HI
Meteorites and sun's heat, 1 1 1

— radium in, 134

Mica, pleochroic haloes in, 109

193

INDEX

Minerals, radium-ratio in, 99,
100

— actinium-ratio in, 100

— lead-ratio in, 141, 143

— helium-ratio in, 143

— choice of, 153
Mississippi as denuding agent,

— bottom load of, 54
Moon, 15, 19, 20, 27, 28
Mose"s and geology, 3, 9
Moulton, 28

Murray, area of land, 52

— salinity of rivers, 50

— volume of ocean, 63

Nebulae, Laplacian, 24, 28

— spiral, 28, 29

Nebular hypothesis, 23-25, 28
Nile, annual deposit of, 81
North America, denudation
of, 50, 56

Oceans, origin of, 25, 50

— mass of, 53, 64 j

— volume of, 53, 63

— saline content of, 53

— age of, 68, 89
Origin of earth, 22, et seq.

Perry, 16

Phillips, 10

Phosphates, helium in, 154

Phosphorescence of uranium

salts, 92

Pitchblende. See Uraninite
Pleochroic haloes, 107-109,

170

Polonium, discovery of, 93
Poulton, biology and geologi- j

cal time, 21
Products, final radioactive,

ico, 140

Radioactive disintegration, 98

physical independence

of, 1 02

Radioactive layer, 133
Radiation from uranium, 92
Radium, discovery of, 93

— detection of, 93, 103-107

— emanation from, 98, 104

— helium from, 96, 138-140

— a-rays from, 96, 138

— half-life of, 98, 99, 179

— atomic weight of, 142

— genesis from uranium, 99

— heating effect of, 96, 101,
129, 131

— end products of, 100

— in earth's crust, 103, 131

— in igneous rocks, 131

— in sediments, 131

— in sun, 117

Ramsay, helium in minerals, 96
Reade, chemical denudation,
50,62

— age of earth, 86

— volume of limestone, 88
River-water, volume of, 53

— sediment in, 56, 57

— salinity of, 53
Rocks, radium in, 130, 131

— thorium in, 131

— chlorine in, 70

— sodium in, 67

Rontgen rays, discovery of, 92

— nature of, 94

Runge and Precht, heat out-
put of radium, 101
Rutherford, age of minerals,

154

— Becquerel rays, 93
Rutherford and Barnes, heat

output of radium, 101
Rutherford and Geiger,
counting of a-particles, 138

194

INDEX

Rutherford and Soddy, dis-
integration hypothesis, 96,
98

— radium and sun's heat, 116

Schwarz, 49

Schuchert, 78

Schweidler v. and Hess, heat

output of radium, 101
Sederholm, annual deposits,

43 .

— thickness of sediments, 43,

Sediment carried by rivers, 57
Sediments, volume of, 65, 68,

— pore space of, 65

— thickness of, 43, 44, 76, 77

— rate of deposit of, 81, 84

— rate of accumulation of, 47

— radium in, 131

— thorium in, 131

— sodium in, 67

— on Continental shelf, 83

— oceanic, 59

— time represented by, 86
Sedimentation, cyclic, 41, 43,

44
Soddy, generation of helium,

radium, 99

Sodium in rivers, 53, 64

— in ocean, 53, 64

— in igneous rocks, 67, 71

— in sediments, 67

— age of ocean, 64

— wind-borne, 66

— in sewage, 66

Solar system, origin of, 23 et

seq.

Solar constant, 113
Sollas, thickness of sediments,

Sollas, age of earth, 86, 87

— age of oceans, 62, 63

— biology and time, 21

— glacial retreat, 43
Steno, 5, 6

St. Meyer and Hess, heat
output of radium, 101

Strutt, determination of ra-
dium, 105

— radium in rocks, 129, 130

— rate of generation of* he-
lium, 139

— leakage of helium, 149

— accumulation of helium,
IS4-IS7

Sun, origin of, 25

— helium in, 95, 117

— radium in, 117

— temperature of, 119

— duration of heat of, 12,

14, 112, 114, 120

Suzuki, age of earth, 128

Temperature - gradient of
earth's crust, 122

— due to radium, 128, 132
Thermal conductivity of

rocks, 123

Thomson. See Kelvin
Thorianite, generation of he-
lium by, 139

Thorium, discovery as a
radio-element, 92

— family of elements, 100,190

— end product of, 100

— heat output of, 131

— distribution of, in rocks, 131
Tidal retardation, 13, 26, 27
Transformation, radioactive,

97, 98

— end products of, 100

Unconformities, 160

195

INDEX

Underground temperature, 12,

13, 122-8
Uniformitarianism, rise of, 7, 9

— criticised, 12, 78, 174
Uraninite, generation of he-
lium by, 139

— lead-ratio of, 151, 158-62
Uranium, atomic weight of,

142

— discovery as a radio-ele-
ment, 92

— phosphorescence of salts
of, 92

— range of a-rays from, 178

— family of, 100, 190

— end products of, 100, 140-
142

- generation of radium from,

helium from, 142

lead from, 142

— heat output of, 116, 131

— half-life period of, 179

— time-average of, 143, 179-
182

Uranium-bearing minerals —
Llano Co., 151, 152
Connecticut, 158, 159,
Carolina, 159, 160
S. Norway, 161, 162
Brevig, 163, 164
Mozambique, 153, 160

— alteration of, 150-153
Ussher, chronology of, 3, 10

Van Hise, 88

Velocity of a-particles, 1 77

Watts, marine erosion, 58

— cycle of deposition, 85
Weathering of rocks, 48, 49

— of minerals, 150
Wilson, W. E., sun's heat,

116

Zircon, helium-ratio in, 156
— lead-ratio in, 160, 164
Zoroaster, chronology of, 3

WILLIAM BRENDON AND SON, LTD
PRINTERS, PLYMOUTH

PLEASE DO NOT REMOVE
CARDS OR SLIPS FROM THIS POCKET

UNIVERSITY OF TORONTO LIBRARY

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Texts

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "The age of the earth"

See other formats